Is It Fair To Keep Male Palties Only?

A lot of people keep just male platies. They do squabble amongst themselves, and you can have problems with this. But as you say, with females as well you'll have a lot of fry. One option would be females only. Yes, you will have fry for a few months as they will undoubtedly be storing sperm if not actually pregnant when you get them, but they will eventually stop giving birth and females are just as colourful as males.

The only thing I would query is the size of the tank. The aquanano 40 is a cube, 40cm wide, deep and long. The minimum recommended tank size for platies is 60cm long. I know the fish profiles on here say 45cm, but I usually go by the data on Seriously Fish.

Do you know if you have hard or soft water? It should be somewhere on your water supplier's website, though whether your kettle/shower head fur up or not will give an indication. It is always easier to get fish to suit your water rather than try to change your water to suit a particular fish.

There are a few people on the forum who have the aquanano 40, they should be able to tell you the type of fish they've kept successfully in the tank. Colin B will also have some thoughts on messing with the filter 
But if you have soft water there are quite a few tetras that will be fine; hard water is a bit more tricky but fish like endlers would be suitable - and you can keep just males as I have some