A. A 12 gallon would only be able to hold a peacock up to 4" max, and I wouldn't even do that except for short term holding. I would go with another mantis. N. wennerae and G. smithii are good choices. P. ciliata is also a great choice if you don't mind spearers. They are very active and their colors can include bright orange/yellow.
B. Silica is a very sharp substrate. I wouldn't recommend it for any mantis tank, especially a burrower such as a peacock or a P. ciliata. The sharp substrate will wear down on the mantis and cause undue stress, as well as make it look bad. I wouldn't suggest it at all. Tahitian moon sand is fine, and aragonite sand is fine. Crushed coral, crushed shells, and silica sand are all substrates I would avoid.
Dan