Hello and welcome to FF! Yes, it's good to vary their diet. Frozen Daphnia, live blackworms, etc. (nothing freeze dried) should be OK. You can use a small feeding plate that will sink to bottom. Depending on how strong filter current is; it could push the worms out of dish. I would not worry about one grabbing another's foot while eating. It's something that just can't be prevented; not that it's common either.
Myself would keep all frogs together; do watch the shy one get's some food if feeding off plate. Would not add anything to an established well balanced tank; specially now that you plan some travel

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