Definitely agree with Bomb. Less not always better! I have 4 plants (going to get more) in my 2.5 and it's doing great. Just beware as many people have said planted tanks are addicting! Once you start you can't stop!
LOVE the idea of a moss wall in a 2.5 that just sounds so pretty!
At the moment I'm just using cheap petsmart gravel. I have a java fern which is doing fine in the gravel (got new shoots sprouting up from the gravel) and a dwarf lily which is doing great. I also have an anubias nana tied to a rock and hornwort floating and a bunch tied down. I let some anacharis float along the top.