I'm sooo confused! I just got Rashad's Talon (using BGT-weidu btw) and it won't let me use it on my Swash. I have the requisite str and everything, what could be going on here? Thx in advance!

Thieves can't wield scimitars.

I've definitely been mainhanding one all game. :P

What race is your character?

She's human. But I actually got the answer already. Something about how ToTSC added in scimitars that weren't covered under the ability of thieves to use it, so I had to install a fixpack and stuff. All is well. :D

The way BG2 implemented the scimitar/wakizashi/ninjato proficiency is strange. Straight thieves can't use scimitars but they can use the others, druids can ONLY use scimitars, and it's not necessarily readily apparent which type the weapon is. (In ToB there are a couple that don't follow the normal rules.) It's also pretty arbitrary, and there's been several patches over the years making all of the weapons in that category usable by every class that can select the proficiency for them.

I can equip them just fine with a swashbuckler using BGT-Weidu (just tried it, cheated them in on a level 1 swashbuckler with shadowkeeper)

You aren't trying to equip it in your off-hand while wearing a ranged weapon perchance?