This is how MTS starts, Lol
A 75gal is only good for one full grown Oscar & no tankmates. So, I wouldn't recommend it unless you want to go with just one fish.
Severum - you could get away with a pair for quite some time... buy 6 juvenile, let them pair naturally, sell/trade off the extras, add some dither fish & BAM!
Peacocks - lots of little fish, which are colorful & active, but not really interactive, with their keepers, unless you're about to feed them & they're begging for food.