who said you cant put tonnes of plants in a2.5? XD
however you need to look for smaller species. I've foun moss, baby tears( not dwarf!) micro swords and a smaller species of cryptocoryne works well for me. you can have taller plants in the back :) you may need to dose some ferts later. it will depend on your stocking :)
here's my 2.5 planted. there are atleast 15 species of plants. I also have anubias nana petite, but am not a big fan of the anubias genus :)
I use a student table lamp and a 23w 6500k light cfl. However I do recommend starting with a 13w.
I dont do much for my plants. the left side has a soil based substrate, and therefore accordingly have planted plants, whilst the right is plain sand (and not very deep at all) so it is all moss on driftwood and free floating susswassertang.
All I do for my plants is dose a drop of flourish everyday :)
thestocking is higher to meet demand for plant nutrients. there are quite a few ramshorn snails, mts, nerite and I recently added a pond snail. I also have 3 large cherry shrimps.
I have eco complete in my one gallon (not completely scaped yet :P, it will be, when i rehome the current betta)
it's sharing the same light with my 2.5