Swashbuckler, slashing grace, scimitars and pirahna strike

[Evilserran]

My swashbuckler uses a scimitar with slashing grace for all his class features. At level 9 i snagged pirahna strike, but it was recently brought to my attention that that may not legal RAW (i.e. pfs character) Does anyone see a way that would work, or did i take an illegal feat, and have to fix it? If you see a way it works, please cite source for me if you please, and if not well, it looks like not is probably the case.

[EvilMinion]

It is not legal, PFS or not, as Piranha Strike only works with light weapons.

The scimitar (and the rapier) are not light weapons... despite the fact they can be finessed.

[Morganstern]

Unable to link it, phone is acting up, but here is Effortless Lace, which once applied to a weapon will allow you to treat it as a light weapon for most purposes, including feats. Just make sure to buy a spare in case someone destroys your weapon and you need to use a backup.

https://www.d20pfsrd.com/magic-items/magic-weapons/specific-magic-weapons/e ffortless-lace/

EDIT: Looking at your post again, I should stress that Effortless Lace isn't allowed in PFS, but is not third party or anything like that. Don't know if this is a PFS question or just wanting a RAW answer, so figured I'd clarify.

[Syries]

No way for a scimitar to be used with piranha strike in PFS, I'm afraid. Get that str up to 13 and take power attack.
Or just not worry about it since you're already getting your level to damage anyway. Another alternative is to get weapon specialization as you qualify as a fighter for all combat feat prereqs.

[merpius]

Looks like power attack is the only power-attack type option for this, but it is worth noting that Dervish Dance is a thing that is probably pretty useful for this kind of set up.