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Yes! Try to get all of one sex, or 1 male and 2 females, if at all possible. They are hard to sex when they are real tiny, but the males will have a bump under their armpits and females will not.
That method isn't going to work in your tank. The guppies and shrimp will clear out the food before the frogs make their way to it. And/or a nasty fight could ensue. If my frogs got to their feeding dish and shrimp and guppies were feeding at it, they'd probably end up being part of the meal!
Personally I would be a little nervous about feeding wild caught fish meat. There is a chance of introducing parasites. Freeze the meat you want to feed your frogs for a day or two before feeding to reduce the risk.
Brine shrimp are tiny, so that's no problem, but you bring up a good point. When it comes to the fish, earthworms, things like that. The frogs have no teeth and no tongue, so they are prone to chocking. Make sure foods aren't much larger than brine shrimp.