I would strictly advice against any mixing of species; there have been several stories and threads around here discussing this problem. Have a look at Species Mixing Disasters if not done already.
A frog being big enough not to get swallowed by an Axolotl (for instance African Clawed Toads) would readily be able to do some big damage to Axolotls.
Pet shops tend to do this all the time to save space and money on housing. Many times the animals don't interact due to bad husbandry, stress or overcrowding.
Housing frogs and axolotls together is a bad idea as someone will more than likely be attacked. There are also few fully aquatic frogs and many of those are chytrid carriers which may infect the axolotl. Most of the aquatic frogs are tropical and prefer higher temperatures than axolotls which means poor housing conditions for at least one animal.
I have my axolotyls in an outside pond and at this time of year the frogs go there to breed. NZ native frogs are rare but Australian bell frogs live here in the North Island. It's really nice to see them together but it is sad to think the axis would eat any eggs or tadpoles. I didn't realise frogs could hurt axis
My axie is acting woerd he was a rescue as stated in a different post he got better and now seems to be deteriorating again...again all tank conditions are OK and as should be. He seems to have some black dapple like pattern under his bottom lip? Is that normal? And his two back feet have a little wierd growth? Almost? Tiny tiny tiny lump. His capillaries in his feathers also appear darker and more visible. Any help would be appreciated