Some of this has already been mentioned, but doesn't hurt to summarize together.
A 40 liter is about 10g, so none of the fish are suitable except the Betta, alone. Betta are not community fish anyway, so if you want just the Betta in the tank, that will be nice, with some floating plants.
Once you know your water parameters (GH or general hardness, KH or carbonate hardness/Alkalinity, and pH--all of which you can ascertain from the water folks--it will be easier to decide on suitable small fish (if the Betta also goes). And if a larger tank is planned, knowing the parameters is still necessary to select suitable fish.
Someone mentioned the profiles, second tab from the left in the blue bar across the top of the page, thee are many species with photos included and each has data on parameters, tank size, numbers, etc.
To your initial queston on the Bristlenose Pleco, they are not active fish much of the time, preferring to browse plant leaves and wood, but once settled you should see them out and about. But this will be a suitable fish in a larger tank.