My electrician ran a new 14g gauge wire to my attic bathroom light, and I notice that it is on a 20amp breaker. Now I want to add a GFI in the bathroom. Can I put a 15amp GFI on that leg before the light switch?

We need to start with the bigger issue, you can not use 14 gauge wire on a 20 amp breaker. This gauge is too light and the whole install is wrong and dangerous

After further investigation that line also feeds wall outlets in 2 bedrooms on the lower level. The wiring is older BX wire that is tied in somewhere but not the panel box. Is it easeir to change the breaker to a 15 amp.

Yes the breaker should be switched to a 15 amp if you have 14 gauge anywhere in the circuit. Sounds a bit squirelly so be sure you know what wires do what. You could call 3 lectricians and get bids. Then know more about the problem and whether you still think you can handle it

At first blush it sounds like your electrician needs to get his license pulled.

He installed a code violation just for you.