I think a cleaner shrimp would make a great addition to you small tank. Either a skunk cleaner or perhaps for a Blood Red would add a cool splash of color.

About the lawnmower, he would probably do well if your tank is developing enough algae for him subsist on, or if you find a particular macro algae that he will eat which you can cultivate in the tank easily enough. Ours have learned to eat flake and pellet food as well, but that doesn't appear to the case for everyone.

In any event it sounds like you are doing things the right way, nice and slow.

and Welcome to CaptiveReefing!