First of all I normally say, let your test results lead you..dont add more fish to a system that is already struggling to hold respectable levels of No3 and Po4 with minimal effort becouse all you will do is increase the work you already have on your hands maintaining that volume of water and its chemistry.
Secondly, plan ahead..Think long term and how much potential load your fish stock will have on that volume as it grows...Bigger fish dont just give out more waste, they also require more feeding so the effects are allways greater than you think over time.
If you want to play it safe, think about adding fish that serve a purpose, ie those that can by and large just potter about doing thier own thing without so much reliance on you feeding them. Some of the smaller opportunistic scavengers and grazers are great for this as they take up and recycle items that would otherwise go to waste and break down...admittedly they give out waste as well, but the net result by way of a small % of assimilation can be a lowering of the overal mass of material thats broken down within the tank as a whole.
regards