Do you need 1500TT to act as PIC for Part 121?

Well this may not be the correct FAR to reference...

§ 121.437 Pilot qualification: Certificates required.

(a) No pilot may act as pilot in command of an aircraft (or as second in command of an aircraft in a flag or supplemental operation that requires three or more pilots) unless he holds an airline transport pilot certificate and an appropriate type rating for that aircraft.
Do airlines have insurence requirements for captains? Like you need 3xxx hours to act as captin in a 737 with a co-pilot that has 1xxx hours?
Yes. The company I work for requires 3000TT and 500TT in the CRJ for Upgrade.

But some carriers will upgrade into smaller equipment (BE1900) with bare ATP mins...
Yes, and check out the new part 91 Subpart K for the new fractional ownership regs. You will now need 1500TT to act as a professional pilots for a part 91 Frac outfit now too.