In my small 3 gal, the rocks came with a few snails (not that I seen them when I was buying the rock, but after about 3 weeks I had small snails everywhere. I really didn't put anything any my little tank for 6 weeks, besides the sand and rocks, the sand was live from my big tank, the water was from my big tank, and the rock was old rock with no die off going on, so I could have started immediately, but didn't.
So if you can be patient enough, you might grow your own clean up crew, or the beginning of one without even doing anything but waiting and letting life happen.