I should note here my only experience with the series is
XCOM 2, so this probably only applies to converting the reboot series.
For weapons, I'd probably have the assault rifles as SCAR-H's (HT122), probably firing APHC (5d(2) pi-). The magnetic rifles would be smaller caliber - probably 4mm - but deal 50% more damage (7d+2(2) pi-). The plasma rifles would convert damage to tight-beam burning and have high damage, but be semi-automatic; I'd say 10d(2) burn. Other weapons would follow the same trend. I'd say the base shotgun would be something in 12G using a cinematic flechette loadout that grants AD(2), the base sniper rifle is an AI AW (HT 120), and the base cannon is probably the XM214 (aka 6Pak, apparently found in
Seals in Vietnam p.65). For the plasma versions of each, I'd suggest they do around the same damage as the magnetic, but deal burning rather than small piercing (the assault rifle gets a damage boost in exchange for switching to semi-automatic; the shotgun and sniper rifle stay semi-auto, while the cannon stays fully-auto). This does have the unfortunate side effect that XCOM needs to produce/acquire 3 types of ammo at least at the first tier (7.62 for the assault rifles and sniper rifles, 12G MF for the shotguns, and 5.56 for the cannons), but that should be surmountable.
For armor, I'd probably go with top-of-the-line TL8 ballistic fibers + ceramic inserts for the starting "kevlar" armor. I'd boost these to TL9 for the tier 2 armor, and TL10 for tier 3. The powered armor I'd use more as exoskeletons (and relatively low-ST ones at that; see
here for a potential design system), but they'd probably replace some of the ballistic fiber + ceramic inserts with metal plate (which weighs more, but gives better sustained protection against multiple threats). The mobility suits (like the Wraith Suit) may have some exoskeleton components, mostly restricted to the legs.
Once you've figured out the gear for XCOM, scale the abilities of the aliens appropriately. I'd actually suggest giving the aliens weaker versions of the weapons XCOM eventually fields (their magnetic rifles might be 6d(2) pi-, for example) - the aliens can get away with using cheaper gear due to their numbers advantage.