Despite statements above, you cant house Male Bettas with fish that bear vibrant coloured fins, the Betta will react similar to the way it would react to another male Betta, nor are guppies the fastest fish in the tank. Just take note of this..
Assuming your tank is the 4 foot long variety, you have a serious amount of options to diversify your hobby now. Personally, I would leave the fish you have in their tank, perhaps move some to the new one after its cycled completely, and diversify your selection. You seem to have grasped onto fish keeping easily by your description above, which is more than I can say for a lot of others.
Tanks that are 4 feet long could house a variety of cichlids, that come in a myriad of colours, patterns, shapes, and sizes.
Take for instance the Heros Severus, or the "Severum". Very, very beautiful display fish in their adulthood while well fed.
Angelfish, again in adulthood, and even through their youth are wonderful on display, though these are more sensitive than your average fish.
Knife fish - Ghostknives and Clownknives are my favorites.
Mbuna cichlids of any type can build a rainbow in your tank.
Seriously you came onto a big tank, you can facilitate much more asthetically pleasing fish now. I'd never be able to stock a tank that big with tetras and guppies, It would break my heart and I would die.