I suggest for guppies, as they are actually NOT tropical fish but rather sit well at 68-78 degrees, unlike our lovely bettas
I still suggest a heater as it will keep it from dropping dramatically during nights, and other situations.
All fish, asides from the betta and maybe a few others will need filters. Filters are also good to have as it will keep your tank cleaner, and for 5+ gallons -safely saying- your tank can be cycled and is easier to house more than just the hardy platies, guppies, minnows, etc during cycling.
Guppies, surprise surprise, kept in a good sized aquarium can be 1.5-4 inches (that's right. 4.) I've seen some huge guppies due to large tank circumstances. Guppies should be kept usually 2 females to a male, or more females to a male. If you get a second male, I suggest having 3 females each to avoid pestering. If you get more than one male, expect more breeding.. competition = breed more. They prefer the top/middle section.
Mollies, I have owned. Both balloon and regular mollies, can fit in 20 gallons... But I do suggest 30 or more gallons to avoid stunting or over-population. They are livebearers so they will breed on their own... I found balloon mollies are friendlier, and are actually unable to cause much damage when biting, then a regular molly. Regular mollies also seem more vicious... and are of course faster than their bent spine cousins, and get bigger - about 3-6 inches. I suggest 1 male to 3 females for them, as they are more persistent than the guppies! Mollies like the top, but tend to eat algae and scrounge for food at the bottom.
Danios are pretty fish... very active, and can get to 3 inches. They are egg layers, so don't expext babies - they'll be eaten (except mine since I moved the adults and found hundreds of babies.... x.x). They need a long tank, or a larger tank which I recommend 30+ unless you get a 20 long. They prefer the top/middle section.