I've got 3 peppermint shrimp in with 8 Black Kuda's the Kuda's get plenty of food. Locally for you try
http://www.aquaparadice.com/ EmilyB gave me the link, they are in Calgary I think.
I think your tonga rock is to thick for the smaller horses but it is really hard to tell. Some things you could use are (cough) artificial or dead coral (cough) can't believe I just said that. Mine are all hitching on LR and Calupera so you have lots of options.