i recently got a cannondale r800 which i think is a '96. it was not kept in the best condition and everything is filthy it is an 8 speed and the crankarms say shimano 105 along with the rest of the parts except for the rear derailleur which says shimano 600.
here are the part numbers
rd 6401 integrated 8 sis
fd 1056
br 1055 slr s
fc 1056

i am wondering if it is possible to get the service manuals and exploded diagrams for them to clean them up and assemble them again correctly

If they're working now why not just clean them up, lubricate, and adjust them? I wouldn't try taking anything apart, though. If it's not broke, don't fix it. I've noticed that even mechanics don't like to take Shimano stuff apart- at least the STIs, and the other parts are pretty much disposable anyway.