Hey Chad. Fires are indeed awesome - you've picked an excellent project if you start messing with them.
Cleaning up other morphs with fires just involves getting the fire gene and the other morph gene(s) into the same snake. You can call it a different name you want (example: pastel fire = firefly) but it's really just two old visual genes being expressed at the same time, enhancing eachother. Fires tend to lighten and brighten with age, which assists many existing morphs in looking prettier as they get older instead of "browning out" like they often do otherwise.
any two snakes (of the opposite sex), both with the fire gene, are capable of mating together and producing a BEL. It doesn't matter if they are also carrying genes for other morphs - two fire genes in one snake equals a BEL - if it has other interesting genetics, it won;t matter because you won't be able to see any coloration on these animals (usually)
i don't know if anyone has done fire/butters, but it sounds like you have an excellent project ideaif you get one of each of opposite sex, you could get a uhm... butter fire (fire butter?) ... in your first clutch.