A 75 will be good for almost any type of lion except the really big boys like the Volitans or Russel's.Lions are not territorial and usually get along with each other, an exception being Zebra lions who dont like each other and other true dwarfs like the fuzzy and fu-man-chu.
Tank mates you need to look out for are fin nippers like puffers and possibly triggers. Lions are all bark and no bite and will get their butts kicked in a fight. I havent tried one but I have heard that dottybacks will launch a geurilla war against the lion as well.
Tank mates that are peaceful do well. Mine do not mess with my mandarins. They cant catch my 6-line and gave up after two days of trying. Any peaceful fish that will not fit into their mouths will be good. Smaller shrimp are a target since they are the primary food source for lions but both of mine get along fine with a big CBS. I have both emerald crabs and sally lightfeet crabs and they could care less. Snails will catch a lions attention but again mine have never bothered them, more of a curiousity thing and lions are very curious fish.
Diet should consist of frozen fish/shrimp/crab, grass shrimp and formula 1. DO NOT feed your lion freshwater feeder fish, you will do horrible damage to its liver. If you dont have access to grass shrimp use ghost shrimp and load them up with vitamins and such. Oh yeah and they will overeat so watch them while they eat.
In my opinion most lions are someof the toughest fish out there. The hardest part is getting them to accept frozen. Eventually you will be able to get them to eat from your fingers, that really amazes some people hehehe.
Fu-Man-Choos are the toughest species to keep because many many time despite the best efforts they will refuse to eat but other than them most lions are not too picky.