3 or 4 ADF's (African dwarf frogs) would be fine in that setup and are fine at tropical temperature.
Make sure you dont get ACF's (African clawed frogs) as these grow way too large and will eat the guppies.
Make sure when you get them that your lfs hasnt mislabelled them, ADF's should have all 4 webbed feet, whereas ACF's will have claws on their front feet rather than webbing.
One problem with frogs is ensuring they get food as most fish will snap it up before the frogs get a chance. There are ways around this such as target feeding them with a turkey baster or syringe if it becomes a problem.
Andy