Typically, you should look to provied between 3-5g per puffer, as, despite their diminutive size, they are rather terretorial; however, if kept alone, a 2.5g could work relatively well (if a little on the small side.)
I highly advise that you do a little research- they're not the most difficult fish, but you have some learning to do.
If i were in your position, i'd go for 1 puffer, an oto, and about 6 cherry shrimp, if the puffer allows.
the shrimp have very little bioload, as with the oto.
that leaves most of the waste room for the messy puffer, and adds extra interest to your tank.
Just don't count on the cherries breeding - or rather, count on any offspring getting munched before you ever even see them. And try for the biggest frickin' cherries you can buy, 'cause the DP's probably gonna hassle 'em!
Actually don't waste the money on the cherries. It may take a few days/weeks but the puffer will hunt/kill them. If anything get a few ghost shrimp just to see what happens. My puffer usually lets them live for a day or so then starts harassing them, once that process starts their minutes are numbered.
I'd also be concerned about having an oto in such a small tank with a puffer too. There isn't much room in there to "hide" no matter how thick its planted and from what I've observed puffers are pretty relentless. If one takes an interest in the oto that interest tends to turn into something more sinister very quickly.
I've only had my puffer in a 2.5g for a few months and its maybe medium planted density wise. He gets live snails and bloodworms depending on what I feel like feeding every couple of days with an occasional few ghost shrimp for something different. Do check out www.dwarfpuffers.com though, its a great resource and will explain most things.
IMO a 2.5g is too small for a puffer. I'd go with shrimp only for that small of a tank. I know the general rule is 3g per puffer, but I'd personally start at 5g for 1. They are cranky and messy and while small, really deserve a bigger tank than that.
Dufus: I also forgot to mention that mine is a male and they tend to be a little more aggressive. I wish I could keep a few shrimp with him.
justonemore21: The tank is really tiny. I make sure to clean spent snail shells and leftovers every other day so there isn't any ammonia buildup. I'll probably end up with something a little bigger for him once I convince the wife that he needs more room. For now mine seems quite content with the 2.5g and he's constantly hunting/exploring and not pacing. I'm making sure to keep an eye on this behavior.