Hi and welcome to the forum!
First off a tank that size give you some serious scope to play with - lots of potential!
Right, straight away take the idea of a siamese fighter in a tank this size right out your head. Big no no. These are delicate fish which actually prefere smaller tanks with minimal water flow. In big tanks these fish actually can become very shy and timid and will suklk away at the back of the tank all day. They don't get on with bigger fish either, often falling victim to being fin-nipped or worse - eaten.
The idea of an Oscar is great. You have the tank for one, and possibly a couple of other largish fish that can handle themselves. Certainly a big ol' plec is very appealing, but there are smaller alternatives that can get to around 12" max that are far more beautifull, and appropriate in the long term. The problem with Oscars is they can be very bullish fish - mine certainly is! Mine doesnt accept ANY other fish other than a 4" L024 Plec currently. He's eaten or beaten any other tank mates he once had and he's not even 6" yet. So in a 4 foot tank i'd say an Oscar and a plec is probably about it. In a 5 foot you can have room to add some bigger dithers, or maybe even another cichlid or two perhaps.
Personally i'd go with something a little easier to care for. A larger community built up of slightly larger fish that mix well and are easy to care for. Oscars can get a little dull at times, certainly if he's the only fish in a tank like mine. The bigger community is a fascinating one. You could have a group of Pictus Catfish at the bottom, say 5 to get a good group activity going. You could get a nice plec that stays around 10" max like a Golden nugget, Magnum plec or Lemon plec - the list is endless! For a shoaling fish Rummy-nose Tetras are amazing, a nice group of 20+ of these would look brilliant, especially when they swim together which mine used to always do. Slightly larger tetras such as Beunos Aires Tetras and Congo Tetras can also look beautifull when in larger groups of say 13 or more in a big tank such as you have. As a main centre piece fish you could have a pair or small group of Firemouth Cichlids. These get to around 6" and look stunning when mature. They are also easy to breed and are fantastic parents to their fry.
There are millions of possibilites to try. Best thing you can do is browse the forum, the journals sections in the various sub-forums is a good start as you can see what we all keep. The fish index gives you a good indication of what there is out there to keep and what can mix with what. Failing that throw out questions and you'll get back answers
Hope this helps,
James
. I own two male siamese fighting fish both in tanks of there own but i dont know how many females to put in with each of them or even if one of them could go in the big tank with other fish. x
. I going to go get the fish for it on thrusday i hope to any way. I got my siamese fighting fish yesterday. even named them lol .
