Swashbuckler with Longsword?
 

Would you allow a perk to let Swashbuckler use Rapier skill with his longsword, getting fencing parries and retreats at the cost of not being able to armor up?

Re: Swashbuckler with Longsword?
 

Yes. That perk already exists in a couple of places. "Weapon Adaptation" in Power-Ups 2: Perks, and "A Sword is a Sword" in DF Denizens: Swashbucklers.

Re: Swashbuckler with Longsword?
 

That sounds like a perk to substitute a sw+1/thr+2, $700 longsword for a sw/thr+1 $1000 edged rapier. I don't understand the "not being able to armor up point" unless you just mean the regular Rapier thing of being penalized by encumbrance unless you buy the Armor Familiarity perk.

Overall no, getting effectively $300 and Striking ST 1-2 plus cheaper weapon upgrades (balanced very fine silvered orichalcum weapon is $16,500 cheaper in longsword form than edged rapier) seems like a bit much for a 1 point perk, plus I'm not convinced that it's physically plausible.

If you want a rapier, buy a rapier.

Re: Swashbuckler with Longsword?
 

Yup, I'm with Julian here. I've allowed [Weapon] Fencer on a host of characters over the years, never broke anything.

Pretty sure it's detailed somewhere (in a Pyramid magazine) but Weapon Fencer is basically:

[Weapon] Fencer, must specialize by weapon, must be a thrusting weapon. May use Fencing rules when using that weapon.

So a Longsword Fencer can fence using a Longsword (and the Broadsword skill), bit not when using a Jo, Light Club, Macuahuitl, Khopesh, Broadsword, Large Falchion, Pata, Shotel, Thrusting Broadsword, Bastard Sword, Bokken, Dao, Estoc, Hook Sword, Jian, Katana, Thrusting Bastard, Backsword, Cavalry Saber, Edged Rapier, or Late Katana.

Nitpicky? Sure, but it's also only one point.