I'm sorry but in my opinion a 2.5 gallon tank is too small for any fish, not to mention a betta. Many people are mistaken when they see bettas in a cup of water at fish stores, thinking that it's OK to keep bettas in such small containers. Imagine if you had to live your ENTIRE life closed in a tiny closet, not being able to move around at all. This is not life, but torture. Even a 2.5 gallon tank is way to small for a fish of the size of a betta. Maybe you could get a tiny frog or two, or a snail, and some small live plants (for the 2.5 gallon tank)? If you really want even small fish like neons, I'd recommend at least a 10 gallon tank for them.
I am so sorry to tell you but even a betta in a 2.5 gallon aquarium is pushing the limit. It is true that a Betta can live in one as it is possible to heat and filter that size tank, but no smaller. They will be less active and not as happy as they would be in a 5 or better yet a 10 gallon tank. There are very few aquatic creatures that will do well in the size tank you are talking about. At the most we are talking about a snail or a couple of shrimp. They make lovely terrariums with a lot of nice aquatic plants and a nice snail or a couple of shrimp. But where the fish are concerned, they just do need more room to move around in. The number of fish you are talking about would need at the minimum 10 gallons of space. (and would be better in 15 as guppies only need to be placed in close proximity to breed unless you get 2 males or 2 females)
If you do decide to get anything aquatic in the tank, please do remember that the tank needs to go through the Nitrogen Cycle before any living things other than plants are added to it. This allows the bacteria to breed that will keep them from dying of ammonia poisoning. Here is an item about it that may help you to understand how it works:
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