Small narrow = 737-300 & 500 & 700
Large narrow = 737-800 & 900 and 757-200 & 300
Widebody = 767-200 & 400 and 777
If you are on the 737, you fly ALL 5 models. You get paid what you fly. senior guys try to bid all large narrow flying. No one flys a whole month of small narrow, you always get some large flying because there are so many 800's and 900's (with more coming). For other pay like vacation, dead head, and training, you get paid a "blended rate". Right now the rate is about 50/50, which means you get paid the average of the small and large rates for all misc pay. This means you get paid more to deadhead on a 737-500 than to actually fly it.
If you are on the "756", you fly all four of the 757 and 767 models. The same thing applies. You get paid what you fly and get a blended rate between large narrow and widebody for everything else.
The 777 is easy, you get widebody pay for everything. The 787's are rumored to be a dual type with the 777's, so they will be in the same group as the "778". You'll get to fly both the 777 & 787.