Seahorse stocking capacity 20 extra tall

[billb]

I fianlly decided on a 20 gallon xtra tall for my seahorse tank. It is intended for dwarf seahorses but since they are so hard to get I may just use it for regularar seahorses (H. Erectus) if I can't find dawrfs. The 20 extra tall has the same footprint as a 10 gallon but is 24" tall.

Could I put 2 pairs of H.Erectus in the tank safely? I was also considering the cleanup crew of a few sexy shrimp or possible a couple of mandarins. So I guess the overall question is:
How many H. Erectus could I safely house in the tank and how many in the cleanup crew?

Thanks