If this is your first addtions as in no corals or no fish I would stick to a couple of each of the following.
2 x Banded Trocheous Snails general algae eaters or 2 x Turbos Snails I prefer Trocheous as they seem to work better for me
2 - 4 Nassarius depends on species if Vibex 4 if Tongan 2 Vibex are quite small these are good for turning sand over and general algae eating
2 x Hermit Crabs lots to choose from most commonly available are red and blue legs I prefer blues as they seem to be more active other options are halloweens pretty little crabs seem a bit slow, Orange Tip Mexican crabs excellent crab not widely available. There are a few more species well quite a few more but I have not seen any in shops
Conch`s are a no no for a new tank excellent cleaners but need a lot to eat
Urchins also need a more mature tank also
Serpent Stars are ok not very attractive imo and look to big for a nano
hope that helps