As long as you gut load the Brine shrimp in selco/selcon, etc they will be nutritious. Overall, brine shrimp are one of the easiest to culture (IMO easier then rotifers). Depending on the size of the horsies you are feeding, you probably will want to feed them mostly napulii. I don't know about seahorses but for my reef tanks I never decapsulated them and I've never had a problem.

Here are some links that may give you ideas on culturing Artemia.

http://www.advancedaquarist.com/2008/3/breeder
http://web.cecs.pdx.edu/~davidr/discus/articles/artemia.html
http://www.brineshrimpdirect.com/Wha...rimp-c119.html