Rubberlips only grow to about 4", but are high waste producers. I wouldn't plan on putting them in anything under 10g, and 20g+ would be best. I have 4 in a 55g and just give the gravel a good vac every week to clear out their waste and they do great.
if i were you, and you happened to like cory cats, i would add a small school of them, 6 or 7. that would put some life at the bottom of the 30g tank. you definitely have the space and you would get to see them in action all the time.