American bullfrog-Should i get one?

[Iratus ranunculus]

i already have a dubia colony so that will work out well but the cage size is a little big for me is one in a 20 gal be ok?

For a juvenile? Yeah. An adult will need something bigger unless you get really really clever with your design (Like constructing an over-hanging land area)

If you dont have space for a forty gallon, I would go with a couple of leopard frogs. Similar care requirements, but much smaller.