1. Don't get a 2.5g. It's really too small for fish. Get a 10g. It's easier to maintain good water quality in a larger tank and it opens up your fish selection a lot. Go larger than a 10g if you can.
2. Don't get goldfish. Their digestive systems aren't very efficient, so more food goes out as waste. Fancy goldfish need a minimum of a 30g for the first fish and at least 10g more per additional fish.
3. Male guppies are bright, hardy, and small. Male cherry barbs are a nice blood red, but I've heard some people have problems with them nipping.