Tomatoes get fairly large; they may try to eat your dwarf frogs or bettas if they see them. They also have toxic skin secretions that may harm other animals in the tank.
Most amphibians need live prey. Some newts can be trained to pellets, but the newts generally available in the pet trade (fire-bellies and relatives) need much cooler water than bettas and dwarf frogs. Eastern newts (Notophthalmus viridescens) are more tolerant of warm water, but I don't know if they will take pellets.
If you want something that will use the upper portions of the tank, and you're willing to buy live food, a green tree frog (Hyla cinerea) is a good option. They're attractive, hardy, and inexpensive, and generally will not bother other vertebrates. They are usually olive brown in the pet store because they are kept too dry; they will turn emerald green in your vivarium, if you keep it nice and humid.