They say about 1" of fish/gallon, but that all depends on how big the fish will grow. Most fish reach their maximum size fairly quickly and stay there if they have the room to grow. My oscar easily added an inch up, and 3 long in a month or two in a 10 GALLON Tank. I figured he would take at elast 3 months to mature into the 55 gallon.
And as for the filter, I find UGF(under gravel filters) to be very difficult. When you have substrate piled onto them and blocking them, it pulls the fifth into the rocks and makes vacuuming the tank unfun. I find mechanical/bio filters that sit on the rim of aquariums to be much more efficient as it allowed easier water flow IMHO. All second hand tanks I have come accross use UGF and I put in different ones. I also no not have much experience with them, but do not hear much good about them