I have the sand in my 15 gallon. The bag (20lbs, I think) was just enough for the tank, although I wouldn't mind having it a bit deeper. It's really pretty, but you do see quite a bit of fish poo if you don't keep up with cleaning. The only thing that irks me much is that it's more difficult to syphon if you're used to using gravel - guess that's a techinique I'm still working on.
I've never quarantined any of the plants I've gotten from a tube, but someone may say differently. All I've ever done is rinsed them off really well under warm water to get off the gel they use for the roots. If you don't mind ordering online, Marimo are much cheaper on eBay.
I use Flourish Comprehensive in my tanks and it seems to work well, but I have no experience with the two ferts you listed.