You're doing it right.
You have fish ,you know what you want ,add what you can in good order.
See your full space even if only building small sections [racks ]..
I learned while building my kitchen ; waste no space ...I have not one "filler board" ,I made every inch go to good use.
With tanks it is more limited but you can mix sizes and turn tanks 'shotgun' or normal .
Look at your full space and plan.