The gender is easy to determine by looking at the underside of the crab. Males have a flap underneath that is thin and pointy, with the point directly below where their mouth is, the female has a round flap that reaches all the way around to the legs on each side, like a half moon. The females belly flap is not pointy.
The males claws are generally larger, but this can be unreliable because claws that are lost will grow back smaller until the crabs have moulted a few more times.
Personally, I wouldnt keep them with frogs, the crabs could sneak up on them at night, mine were very paceful though and would only pick at fish that were already dead. Others havent been so lucky, but I've had over 8 red claws and haven't had trouble with any of them.
Try to keep one male with two females in a tank at least 2ft long, and make sure there are plenty of caves and holes for them to climb into, or they will fight for the best ones. Although they survive alone, and underwater they do best in a small group with a land area big enough for them all to hang out above the surface. If you give them gravel or sand, they will create little homes in it and re-arrange it every so often to suit their needs.
They also like having bogwood to pick on for tiny organisms and algae. One of the best things you can give your crabs would be dried out, brown beech leaves, ones with no green left. These leaves appear to have benificial bacteria and substances that the crabs thrive on, they will carry a leaf into their home and chew on it for hours.
Feed them bloodworm, hikari tablets for corydoras cats, or tetra mixed tablets for catfish and crabs (the red ones are for the crabs).
Ken