I have a 20 gallon tank with half land half water
In that tank i have 1 chinese warty newt, 2 firebelly toads, and about 7 guppies
They have all been happily living together for a while now
I think that mixing species is fine as long as you do your research and have enough space for all the animals
As for fish if you have a large enough water section guppies should be fine