I'd be shocked if you find any prop firm that trades through IB. IB, to my knowledge, doesn't license their software out for accounts that don't also clear through IB...and I don't know if they even accept non-retail accounts.
FYI, IB does white label the TWS and/or allow firms to provide their own front end via the FIX gateway. I can't name any of the firms that do that though.
I'm pretty sure IB has non-retail accounts. You can open corporate, partnership, etc. account with almost every retail broker.
I think the real question is "will IB provide increased (i.e. >retail) leverage to institutional accounts?" If not, they are unlikely to attract the typical prop firm that requires a more flexible risk-management model.
Think or Swim's front end is called Liquid, which they developed themselves...IB is their competition.