I need to post up a picture of my updated 2.5 gallon and 5 gallon.... the 2.5 gallon has my favorite decoration EVER.... a really cool log with five holes plus fake plants attached (3 more fakeplants in the 2.5 gallon an 1 moss ball)
the five gallon has two hidey-holes, live plants and fake plants
I think it'd be okay :) My betta has been from his 1 gal, to a 5 gal, back to the 1 gal and now in a 2.5 gal. He freaked in the 5 gallon and seemed scared, so I put him back in his 1 gallon, where he's been for a couple months, then I found my old 2.5 gal and decided to try it. Put him in and the next day he's happy as could be with the biggest bubblenest he's ever built! So it really just depends on the fish :)
I agree with most everyone else. My betta, Ciel, was in a 2.5 for about six months and loved it. He used to swim laps around the tank when he was excited, and when I'd come home from school to say hi. He never tried jumping out, or had any issues in his 2.5 bowl. Then I upgraded him to a 10 gallon, and he hated it. He hid a lot, and when he wasn't hiding, he was darting around, trying to avoid the filter. That's when I downgraded to a 5 gallon. He loved it almost as much as he loved his 2.5. Some fish prefer less space. They're like humans, kind of. They don't like cramped spaces, but they don't like having too much space all to themselves, unless you slowly upgrade them.
My sisters' fish are in 1.5s right now, but once I get my new 10 gallon (planted with black sand) for Ciel, they'll get my old 5 gallon to split. So I definitely don't think 2.5s are bad. As long as the fish are comfortable, I don't see what's wrong with it.
I am so glad I don't live in a city where these cups are common practice. I know it is temporary, but even my big shops manage at least half a litre to a litre, with a drip filtration system and/or heating, glass compartments, not cups where they can be picked up by passers by and shaken around.
2.5 is fine. Just make sure it still gets proper water changes and heating. I personally don't like downsizing fish from tanks if I can help it, but that depends on each fish and how they react.
If you're buying new, I'd consider getting a 10 gallon instead of a 5 gallon. Around here at least, they run only a couple dollars different in price, and with a 10 gallon you get twice the space for about $3 more.
I would not make such a jump. I so far hace a .8 g, 1g, and the 3. I slowly introduced Lakitu up to what he has. It oonly makes sense to gradually go up. If I plunked my confident little fish into a 10g he would likely hide all day. If I put him in 5 g, 8g and then 10 I feel he would be okay.. (ps and then you have all those extra tanks lying around) It's a comfort thig for them. They either come in tiny bags/cups and then live there util they go home. No wonder so many freak out in large spaces. I gues that's where the myth comes from.