First off: A 2.5 gallon tank = 1 betta only (bettas don't really like company anyway!).
Planting a small tank is a great idea. It improves the water quality (if it's growing well), looks beautiful and gives your fish a sense of security and somewhere nice to sleep.
The biggest challenge I see with a 2.5 gallon planted tank is finding the right lighting for it. You'll want your lighting to be about 5 watts/7 watts maximum of fluorescent lighting directly over the tank (which equals 2 to 3 watts per gallon)--any higher and you run the risk of developing algae problems unless you're injecting C02.
I have seen little clip-on fluorescent lights at Big Al's and online, (usually used for open-top tanks) and I've known people to just shine a CF desk lamp down onto their planted tank (which might be covered with acrylic or glass so the light can still reach the plants). The great thing about the glass top method is that you can totally control & adjust the lighting to your own tanks' needs. I don't know much about hood lighting for a 2.5 gallon, perhaps others can suggest something.
If you stick to low lighting (say 5 watts, more if your lighting is raised above the tank top), you could easily grow plants like a dwarf anubia, a bit of java fern or java moss (a whole plant could be too much for a little 2.5 gallon tank), or a small cryptocoryne. These plants are quite slow growing, and are easy to care for, with few demands. Try not to overcrowd your tank with too many plants/ large plants/plants with large leaves/fast-growers, or you'll end up making your tank look very small, and you'll cut down too much on your fish's swimming room. Don't worry--one nice, healthy plant would still do wonders. With a tiny tank like yours, more is not necessarily better. Think small.
Plain gravel is fine with anubias, java fern & java moss, since they are usually tied to rocks or a decoration (roots not buried in the gravel). A crypt would still do fine in gravel if your fertilize the water with liquid fert. I use a little 'Flourish Excel' carbon fertilizer every few days .
