So brassicas follow legumes, umbellifers and chenopodiaceae follow the brassicas but where oh where do the lettuces go?

Anywhere I can fit them
I've never seen any disease problems with lettuce.
I'd generally keep them in the more nitrogen-rich part of a rotation since they're hungry sods, but aside from that...

Lettuce is normally grown between slower growing plants (say broccoli). They require soil rich in humous and they love moisture. You should test your soil since lettuce may require lime added as a soil pH of 6.5 is just right (don't need to be too strict).