Hello elura,
My thoughts:
I really don't think you have to worry about 2" and 3" -adult- tank mates .
( if that is is size difference? )
If you bowl feed...perhaps use 2 bowls for a while to make dinner easy for the smaller one.
Just keep an eye on them.....as should be with the addition of any new tank mate of the same species
...to be sure he is not being bullied in any way.
As well......observe closely to be sure your smaller one is not stressed by the size difference.

Having said all of this ( you used 2" and 3" as an example? ) I DO think you should consider age and size differences.
Which, will also come with differences in behavior.

Very young frogs need to remain in a grow-out situations to gain their 'froggy' confidence.