I have a few cleanup critters in my fuge to keep the house up.. a couple urchins, hermits, and a snail or two, so you can certainly keep critters in them.
But andy makes the real point here... a refugium is just what the name implies. A safe haven for beneficial flora and fauna to reproduce so it can have it's benefit.
I have used a fuge though as a place to separate an aggressive fish. Make no mistake, it's not a quarantine tank, but it can be an isolation zone.