Calcium, Magnesium, and a stable alkalinity are all VERY important when keeping macroalgaes and certain types of seagrasses. No need to dose nitrates if you have a reasonable bio-load. There are types of macros that only need calcium, high light, and good water movement to proliferate (halimeda sp. being one of them). You might also want to look at seahorse specific forums to glean as much info as you can from them. Seahorses and marine plants go hand in hand!
Also, you might want to look at creating a biotope using only turtle grass (legally collected of course). There are great number of species that live in turtle grass beds that rival some of the reef species in beauty and oddness!
Many people already have planted marine tanks. They are called refugiums. Generally they house specific types of fast growing macroalgaes like chaetomorpha sp. to suck up nitrates and phosphates that would damage the corals in the main display. And yes, caulerpa do go "sexual" ( they release gametes into the water column) when conditions are not to their liking. Caulerpa need to be pruned regularly too and they need sufficient available iron in order to keep them in top condition.
Go to Reefcentral.com and have fun!