I have decided to get an oscar (or two) and i am having trouble on deciding on which ones. I kinda like severums and jack dempseys but bichirs are my favourite! could i keep and oscar with a bichir or a ropefish?

not with the aquarium you are thinking about getting, both these fish can exceed a foot in length

I would strongly advise you to get South American tankmates. These fish will prefer near enough the same sort water conditions as the Oscar. This was anything wrong with mixing fish from different parts of the world, you just got to make sure you're not mixing fish that prefer something completely opposite of an Oscar

so what is the absolute minimum for a bichir¿?