It depends on if it is a 20H or a 20L. Most folks get 20H, so I'll start there. You can do Ottos but they have to be fed, so you will have to decide what type of food you're going to feed. I like using natural food like blanched peas, green beans and things of that nature. The guppys should be no problem. You need to consider if you want to have fry or just want to go with males only. If you have a female she is prego trust me. Kuhlis need at least 5 friends and the tank may be too small once they are adults. Tetras, there are several types that can go with the guppys and betta. Penguins, Black Neons, Lemon, Head and Tale light, Glow Lights. Stay away from Serpeas and the like as they will fin nip.You may want to look at the rasboras, any will work. Also a school of 8 Cherry Barbs would look great as well especially if you mix regulor and albinos. As for the frog, here again make sure you're equiped to feed it properly.