Hello Jordy and welcome to this Forum.
I think the fluval edge small tanks are ideal for keeping paros – I have myself been wanting to experiment a little with them. I would think the size is really fine for a pair, in particular if you have successfull spawning, then it does give more space than the smaller tanks, and you can keep the fry with the parents for somewhat longer.
I am curious though as to how much flow (or rather how little flow) you can adjust the filter to give, – although paros do not mind a little flow, it could also quickly become too much in such a small tank, – so if you start it up, let us know how it works. Theres always the option of blocking the outpour a little with a bit of foam or something, if it gets too much. And yes, I think its a good idea to filter over some peat or something like that.
For gravel, I would not really recommend sand or most gravels, – they all tend to work against a low ph, – the best results are when you are using either no substrate or substrate such as peat, leaves or I have actually started using fluval peat granules, – thats only to cover the bottom so as to make the tank darker, not a big layer, or fluvial shrimp substate which is something like the peat granules. Then add lots of peat, leaves and possible java moss. You will not be able to grow much plants if using no gravel, but plants such as javamos, javafern, ceratopteris and other will do fine.
As for caves, my paros just really prefer the clay caves which you normally would use for small catfishes, – or just any clay pot, broken or half or a piece of it, as long as they can get underneath them. Even leaves will sometimes work, or a place underneath a piece of wood. But the long cat-fish caves are my favourites