congrats dude, I love the 90 gal I was gonna get that but then my dad helped me get a 125 so that's what I got now. in my tank I have a male jag,male mayan cichlid, a male Dempsey and firemouth. all 5-8 inches in size. they get along great.you could get whatever you want in a 90 gal. if I were you I wouldn't get an Oscar. if you want something bad a** I would get something from the parachromis genus if you know what that is if not do some research and youll find out. you could get something from the nandopsis genus like a Cuban cichlid (I have one that's 3 inches in a growout tank so I could put him in my 125 one day) the list goes on. you could get a grammodes cichlid (bad a** fish) you could get a couple dempseys. just make sure if you gonna mix and match put the more ruthless species in last and make sure they are the smaller more submissive fish when put in. for instance my jag was 1 inch when I got him and my Dempsey and mayan were like 4 inches so even though the jag is the same size now hes still unsure of himself in other words he shi** himself when my Dempsey comes around him. u shud be very excited, you just opened up a whole new door into the hobby because basically the choice is yours on what you want.