Don't buy riccia. It needs very high lighting and co2 and it will be a waste of your money if you can even find it.
To clarify a few things:
ADF's will NOT usually tear at a bettas fins unless they aren't fed enough. It is not an aggressive behavior. It is as a result of their poor eyesight. They see something wiggle in front of them and if they're hungry they'll bite.
A 25w heater can and will work in a 2.5g. I use the Visatherme adjustable heaters and they work a lot better in my experience than the 8w - most of which are not adjustable.
Bettas actually prefer warmer water than most other tropical fish- they're comfortable between 78-82F
Guppies and Bettas are a huge no no.
No more than one betta in a 2.5g tank. Maybe a snail. Just one. Even at that there is usually not enough surface area to sustain its nutritional needs.
They also make heating pads for small betta bowls that you could use if you find them.
It is fine to use gravel
and a filter. Get something like an azoo palm filter ($6) that doesn't take up internal space since it's an HOB -
www.drsfostersmith.com.
When to do water changes on the tank and how often? Monitor it the ammonia/nitrite/nitrate levels and go from there.
Jacblades - I mean this in the nicest possible way because I know you care about your animals....(Unless you bred them and sold them...) Do you think it says something about the betta conditions of yours if you've kept hundreds of them? Surely you do not still have hundreds, so I assume they've died?