The pleco will
be going to the 120 gallon as soon as it is set up, I keep telling my
husband to wait, (having nightmares of it going through my floor) I thought
about putting my African dwarf frogs in with one of my betta's, I have read
people have done it, and others say no...

If you're worried about the 120-gal aquarium weight (I'd worry too ),
why not fill it partially? You could grow plants up out of the water,
and even on some dry land area - nice for frogs? If there is dry land
and water, I think it's called a paludarium(?).