Hello and welcome to FF! Due to different frogs requiring different environments and the fact that many produce toxic substances in their skin that contaminate their water and substrate, frogs are best when kept among their own.
In the wild, animals can get away from each other to avoid stressing each other out with their presence. In the typical home enclosure that is impossible and over time, the stress affects the frogs immune system, leading to disease. There are instances of lucky individuals keeping Toads species or Tree Frogs mixed; but there are uncommon and many of us do not recommend it.