That would be entirely your choice, but personally I prefer square or rectangular tanks.
They don't mind what type of plants you use in the tank. As long as there are lots of hiding places for when they moult and are about to drop shrimplets. The thick growth that mosses provide is a great place for them to hide in. And when the shrimplets are born/hatched, they usually hide in moss and find food in them.
Yes that should be fine. Cherry and either CRS or Pintos can be in the same tank without interbreeding.
I wouldn't recommend any fish with shrimps. But if you decide to go down that path and are aware of the risks with fish & shrimp, then go with small fish. Try to introduce the fish after the shrimp have had some time in the tank and have found their hiding spots.
Here are some examples of possibly compatible fish.
Pseudomugil Gertrudae lampeyes,
Dwarf rasbora - Boraras maculatus,
Celestial Pearl danios,
Ottos,
Endler guppies,
White cloud minnows.
But research the parameters the fish require and match them to your tank's parameters.
There might be other fish that people can recommend, but the list is not a guarantee that the fish won't snack on shrimps.