Forgive me if I am not understanding correctly but I do not think it would be wise to mix Xenopus and Hymenochirus; tropicalis is more closely related to hymenochirus but I think it's predatory nature is more closely related to Xenopus. Which means, very good chance an X. Tropicalis would eat an african dwarf frog given half a chance.

Unless I am way way off on the size of X. Tropicalis it should still be considerably larger than a dwarf frog and I am almost certain it will be more aggressive, powerful, and far more predatory.