I've got a 29 biocube as well.
I added the following a few at a time during the first few months i had my tank setup.
7 branded trochus snails
5 nerite snails
4 cerith snails
a dozen tiny blue leg crabs
around 30 dwarf cerith snails
9 or 10 nass snails
And a fuzzy chiton
I had a couple of other crabs that started getting big and killing snails. Next time I will avoid the crabs in favor of a bigger snail variety.
These guys do a great job geeping the rocks clean, and I hardly ever have to scrape the glass.