I'm with lisie about foods for bettas; freeze dried foods, including flakes, can contribute to swim bladder disease in bettas, and increase the chance of constipation - which in fish, can be fatal! My suggestion would be a high quality pellet like one of the brands mentioned, or even multiple brands for greater nutritional variety. And frozen or live foods are great for bettas, esp. finicky ones; blood worms, brine shrimp, midas shrimp, etc. all seem to be loved by bettas. And of anything you can feed, they're the closest to what bettas would be eating in the wild, and as thus, are least likely to cause GI problems. Just make sure if you feed live that the source is good; bacterial contamination of live foods is sometimes an issue.
In a 2.5g tank, you could safely heat if need be. Either way, def. have a thermometer on it, as smaller tanks tend to heat up and cool down quickly. Bettas are pretty hardy about temp, but as mentioned, prefer 75 to 80 degrees. I've heard of them being "ok" in anything from 72-84, but those aren't really ideal ranges.
As for Petsmart... eh. They're "ok." I find their bettas to be at least a little healthier and better cared for than those at Petco and other chain stores, but that doesn't say much. As everyone mentioned, go for a healthy one; signs of a healthy betta would be a fish that is active, agressive, brightly colored, and in perfect fin and body condition, with a nice, clean cup. Make sure there are no lumps and bumps, white spots, fuzz, fin degradation, etc. If possible, go for someone with a bubblenest. It might also be good to go maybe 2 days after the newest shipment; the longer the fish is in the store, the less healthy it will be, and by 2 days, the ones that were going to die from shipping stress are generally gone.
Good luck, and you had better post pictures when you get him/her!
