DIY wee sump/refugium

My main aquarium is a 70 gallon Fluval Osaka. When I first got it a friend of mine convinced me not to get a sump even though I had been set on having one for my freshwater setup (I like having the stability that the additional water volume adds, the ease of adding water lower down, putting equipment into the sump, LOTS of biological filtration, etc.). Having been talked out of this, I ended up with a stand that is not really conducive to a significant sump/refugium, but that got me thinking: what if I used one of my spare 5 gallon aquariums to make a small sump/refug., maybe put a vegetative and/or trickle filter on top, etc. My concern though is that a 5 gallon sump will simply be too small to operate with any stability; in other words, will it constantly be a struggle to maintain a sump this tiny? Will there be other problems?

Thanks!