They'll work fine. Not as well as a liquid, but well enough that you'll know if you have any problems.
Are the plants plastic, silk or live? Betta's are known to rip their fins on plastic plants so live or silk are preferred.
I find my 5 gal for my Betta is a perfect size for him. 2.5 gal isn't something I'd put a Betta in unless I had too. But compared to the cup he probably came in 2.5 probably feels like an okay size to your betta.
Anyways, going back to some of your earlier posts there are several ways to ensure that your Betta is happy. Step one is keeping the tank clean, if the tank isn't clean than no matter what you do your betta will be unhappy.
You could add toys of sorts, giving him things to swim through and explore will keep him happy, another option is to add a ping pong ball, he may enjoy pushing that around the tank.
My Betta loves to be fed peas (I buy frozen ones and put them in boiling water and then shell the peas and cut them up really small) he loves to carry the little pieces around and eventually he eats them.
If he blows a bubble nest it means he is happy (Though some say they make nests anyways) (And I say it depends on the Betta)