Without knowing the capacity of the mini, it's hard to say.
However, if it were me, I'd pop the 5-15 filter on, and check the water levels every day to make sure nothing goes way up. Also, limit your population of fish until you get a properly sized filter on the tank.
I've read that the tank size a filter is rated for is actually higher than it should be. So, for a 20G tank, try to get something that's rated for 30G. That should do well, as long as the water isn't too turbulent.
I wouldn't recommend getting a bottom feeder unless it's needed. I've seen too many tanks with starving plecs and otos, where the owners figured "oh, all tanks need a bottom feeder/algae eater" and got one (or more) without realizing that they actually need a population of algae in the tank to feed the animal :/