Most macros are not going to be picky about the substrate they're rooted in. You could always work in some refugium mud (commercially available) if you'd like. The available iron might be of some help. We've been going back and forth on the topic of substrate in this forum lately.. unfortunately its just too new to know anything even anecdotally from other hobbyists.
Lots of macros actually do not need high light levels. The 4-5watts quoted above would be sufficient in many situations to support the much more challenging seagrasses. Even ambient light is enough for Caulerpa prolifera and Chaetomorpha. As a general rule we're finding as a collective group that most fleshy reds need the lowest amount of light, then greens which adapt to wide ranges, and the browns - like Sargassum, and the coralline reds need lots of light, approaching the 4-5 quoted above.
The easiest species, in my experience, have been Caulerpa sp. members, Chaetomorpha, Ulva, Halymenia, Halimeda and Udotea. I havent attempted Sargassum, and only had passing success with Gracilaria.
Please do feel free to visit the stickie in this forum called "forum favorites". As a person with a planted tank background I think you'll really appreciated some of the threads (overdose phosphate thread and nitrate dosing threads in particular) as well as the CO2 dosing thread that are linked in that post. It will probably help to answer quite a few questions too.
As far as planting.. many macro's are associated with hard substrates, like rocks, and should attached with either frag glue or can be rubber banded in place. They will eventually develop holdfast tissue that will root them into place. Some are sandbed associated and can be stuck directly into the substrate, much like bunch plants from freshwater. A few are midwater to upperlayer floaters (not unlike duckweed, frogbit, etc) which can be set in place where you like or left to float.
As a tip: sometimes the best way to get really diverse macros is to buy the green leafy live rock from dealers. Dr. Mac's Aquaculture has a box set I think of 'green' rock like this. Reefers dont generally like it because it needs to be cured of the green life, but thats the goal for us.
Downside: potential introduction of unwanted macroalgae, perhaps say Bryopsis (hair algae).
In all, I think marine planteds are more than a little easier than freshwater. More forgiving with algae outbreaks at the least.
Any other questions? There's a growing number of us on this forum that dig marine planted tanks.
>Sarah
