You should move your guppy fry to another tank or get the betta a 5-10 gallon of his own. Bettas and guppies, espescially male guppies with their long bright fins, do not generally get along at all . The betta is quite likely to attack and even kill the guppies. Since they are fry they may also get eaten or chewed if he mistakes them for food.
A low flow filter with a spray bar attatchment, or a sponge filter is good for a betta, and of course a heater is needed set to around 27/28 degrees C.
You will need to plant the tank quite heavily with live or silk ( not plastic) plants ( or a mix of both ) as these fish like a lot of vegetation to rest on and lurk in. Tall plants are a good choice so the fish can rest on them near the surface and build his bubblenests around them. Surface plants are also a really good idea. Frogbit, riccia, water lettuce and even duckweed ( as long as you regularly remove some to keep it from taking over the surface ) are all good ones to try.