Rio 50 is nice and small. I use it in my 12 gallon and it does the trick. Its not noisy like the other RIO but after a year the suction cups tend to give out pretty easy. Mine have. You could also try the rio 80 or 90 i think it is. Slightly more GPH same size. A little louder but not noticably. Neither is very loud to begin with. You will not be able to hear them over the filter at least.
i have had luck with the aquaglobe power heads, they come in 100, 200, 300 gph and are adjustable and 360 degree rotatable
I have one in my 5 gallon pumping into my fuge, and one in a 20 gallon long working a cpr overflow. These P.H. are very very small
hope this helps!