Normally I'd say this is a good way to do it as many people don't get tanks large enough to keep the fish they want, but M Mopeytitan is planning for the fish he wants. It seems to me he is doing it the better way. He is asking for the requirements for the fish he wants and planning the tank from there.
If I wanted a RTC but didn't know about them I wouldn't go and buy a 55 then ask about putting one in it. No, I'd ask tank requirements so I can buy or build something proper for the size it gets.
M Mopeytitan , keep asking your questions. It is good you are researching the fish you want and their requirements first.
Thanks, my main concern is that I want Arowana specifically Asian and hope to keep 3 pairs, colours yet to be decided. The Rahs were sort of an add on that I thought would set the tank off nicely. And have read that it's ideal to have some sort of bottom feeder with Aros.
If it turns out I can't have them then I can't have them. I know quite a bit about the Aros already it's mainly the Rays I'm worried about