A rough guide is 1 inch per gallon so for 75l (16 gallons) you're looking at 16 inches of fish. But this is adult size not the size they are now and not all fish are the same, for example an inch of a tetra produces less waste than an inch of pleco, so the more pleco's you have the more filtration you need. It's always sensible to have more filtration that you think you need especially when you have plecos. You're L134 and BN will take up 11 inches alone so you only have 5 inches to play with, however if you stick to something like small tetras you could probably have more than 5 inches of fish, but one more hypan and that will be the lot.
Yes just test your tap water for nitrates with your test kit.
