[Space] Interactions between species

[Varyon]

A lot will depend on what technology - and particularly what superscience technology - is available. Handling different atmospheric conditions would involve sealed-off sections of town that replicate a given species' native atmosphere, and visitors would need appropriate gear. Aliens wearing respirators is a space opera staple, really, and wouldn't be out of place. This won't interfere with the "aircar" requirement; for the "cantina" one, there are a variety of options. Some aliens may have separate orifices for breathing and eating/drinking, so they'll just need the breathing tube(s) in the respiratory orifice(s) and can otherwise behave normally. A character could have cybernetics that let them breathe the air without much issue. Cantinas may provide (or the character could bring with him/her a collapsible version) something like a clear box that rests on the table and goes over the character's upper torso as a "feeding chamber" of sorts - sit with your upper body in the box and you can remove your respirator and enjoy your refreshments alongside your xeno buddies. You could also have an environmental suit that allows for eating and drinking with it on, although eating and drinking from a tube isn't nearly as fun as sharing some tacos with your friends.

It's a bit more problematic if you are reliant on a specific atmosphere in a manner other than "this is the mix - and partial pressures - of gases I prefer to breathe." For example, many amphibious species need to stay moist, and thus need a humid environment. In such cases, there may be some fairly basic workarounds (maybe the amphibioid carries around a spray bottle and spritzes itself from time to time), but you may need to just use a dedicated environment suit.

Handling different temperatures is similar, calling for either environments set to a comfortable temperature for a given species or an environmental suit to be comfortable in another species' favored temperature. Although honestly, given everything is cabon and water based, you probably aren't going to have many species that are evolved for temperatures humans can't handle without advanced technology - simply stripping down to a swimsuit or bundling up in heavy winter clothing should be sufficient (although if you'd rather not be - or legally aren't allowed to be - nearly nude or heavily-bundled, an environmental suit with built-in air conditioning and/or heating may be a better option).

Handling different native gravities is where superscience would be really useful, allowing for a localized gravity/anti-gravity field. Absent that, high-grav natives should be able to handle lower gravity without too much difficulty, although wearing weighted clothing wouldn't be a horrible idea; if staying long term, you may need a strict exercise regimen and/or treatments to avoid low-gee tissue degradation (mostly bone and muscle loss). Low-grav natives in high-grav environments are in more trouble. A small difference they can probably overcome without too much difficulty - in many ways it will be like walking around with weighted clothing. A larger difference will probably call for a mobility aid, such as a wheelchair or similar, and they'll want to be careful not to overdo things to the point their circulatory system can't keep up. A tank of liquid with legs on the bottom might not be horrible either, letting the character float, but at that point you're probably beyond the region where aircars and cantinas are an option. Extreme differences basically mean the low-grav native cannot be on the high-grav planet (or high-grav region, for a station with spin gravity, or if there is artificial gravity but the technology cannot be miniaturized for an excursion suit). Appropriate exercises, treatments, cybernetics, etc may be able to allow for a low-grav native to function as though they were a high-grav native, but that's not exactly something you can toss on to go to the local pub.

Morphological differences interacting with gear is another place where superscience is useful, as you can have gear that basically shapeshifts to meet the requirements of the user (of course, superscience isn't strictly necessary for this, but it makes things a lot easier). Absent the "morphing gear" option, you'll generally need different gear for those with different morphologies. This will mostly be the case for clothing, armor, and similar. For tools and weapons, having the right kind of fine manipulators is ideal, but often simply having fine manipulators will be enough to use the item, albeit sometimes at a penalty. For example, an item designed for use with tentacles might have multiple small gripping points, for wrapping the tentacle around it; a human may need to use both hands and still wind up with an awkward grip. You might also have some gear that's unusable without modification, or maybe just without using additional tools. A species that has retractable claws or similar might have a device that is activated by extending a claw through a hole to press a button; a human would need to hold it awkwardly and shove a thin piece of metal through the clawhole to activate it. A four-armed creature might have a tool that requires four different sections to be manipulated at once; a single human might be able to manage using both feet and hands, but more likely you'd need two humans working together.

EDIT: You could also have an issue of morphological differences preventing mobility. A purely-aquatic species is going to have trouble getting around on land, while a landbound one won't swim too great. The former would basically want the same types of mobility aids as a low-grav native; the latter may want flippers and/or some automated method of propulsion to swim more readily. In both cases, basically piloting a small mech designed for the environment could be an option.

Different nutritional requirements, at least, are typically going to be easy - characters carry different rations, restaurants and the like often have "xeno-friendly" menu items, etc. The bigger concern here may be cultural. Delicacies for one alien species may be absolutely revolting to another, while herbivorous species in general may well object to the consumption of meat. PC's who are working together are probably cosmopolitan enough to accept this, so you won't have the herbivore PC throwing a fit every time the obligate carnivore PC orders a steak (or when the omnivore PC does the same, although they may get a bit of side-eye) - but that obligate carnivore PC may have trouble finding food in a cantina in the herbivore district, and may not be able to bring his/her own (although the herbivores might be alright with insect-based foods, like the carnivores eat in Beastars).

Different sizes can be problematic if they vary tremendously, although as dcarson notes, having a lot of things scaled to the larger species makes things more workable. An ogre is going to have trouble fitting into a halfling-scaled pub, but a halfling at an ogre-scaled pub just needs a booster seat, and maybe should bring his own mug (or just a straw, and maybe a friend to split that gallon of ale with). Naturally, gear for characters of different sizes will need to be scaled to that character's size and strength, so things are unlikely to be interchangeable, even if going with the "morphing gear" option above. Highly-cosmopolitan cities will probably have most things scaled to be able to accommodate the largest species, but with options available for the smaller species as well. Using my ogre vs halfling example, the building will be large enough admit an ogre comfortably, but will also have halfling-scaled amenities - the pub will probably have some halfling-sized tables in addition to the ogre-sized ones (and booster seats for halflings who opt to sit at an ogre-sized table), and will also have cups/glasses appropriate for a halfling in addition to those meant for the ogres.