I would try to get your alk up a little more (I am assuming 4.5 is dKH)? If so, with an alk that low, your ph is going to swing like crazy. I'd get some 2 part supplement and just dose a few ML a day (5 ml or so) of the alk part till your closer to 8-10.

As to a cleaning crew, for a biocube nerite, cerith and nassarius are good snails for the tank and won't get really large. I'm also a fan of a few blue legged hermits as long as you have some extra shells for them to go to.

Do you have any algae or diatoms growing in your tank yet?