Congrats. on the tank Puggy
Since you originally put this thread in the tetra forum, is that the type of fish you are wanting...tetras?
I'm a bit biased and not a big fan of mix and matching species, I prefer a species specific tank. Maybe I'm just lazy..at least then you don't have to worry about compatibility issues etc. and can concentrate on that species specific needs. I think a large shoal of one or maybe two types would be very nice and that way you will see and allows them to show their true nature and shoaling behaviour which we often don't see when they are kept in much smaller groups.
Propably easier if you say what type of fish you were interested in getting and then we can help more. From the look of your discus/anglefish tank )beautiful pic. BTW) it looks like you already have neons and black neon tetras. You have a bit of time to think about what you want, while you are getting the tank up and running.
Just an afterthought....if you think any of your other tanks are overstocked at all, you can move some of those fish into the new tank and then increase their numbers to make a bigger shoal.