I don't think the size is a big problem .
I'm sure someone will correct me if I'm wrong
I would try to get the closest size you can
Retf do prefer to have another . They live well in groups
I would wait till the one you have is really settled in first .
Quarantine any new frog in separate tank for a couple months .
Make sure there both eating , pooping , and watch for any problems .
Keep the tanks away from each other . Then after you know the newer frog is heathy and acclimated
Putt the tanks next to each other a couple days . Then add new frog and watch for signs of stress .



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