No need to quarantine in separate tanks. They have been kept together so far and if one is ill they will all have the same bacteria or parasites.
As for keeping them together, just make sure that when you put them in with your older MHD that they are well enough established to not be competing for food. Sometimes if you have a larger baby and some much smaller ones the larger will get most of the food leaving little for the younger ones. Otherwise you should be fine to keep them together once they have a clean bill of health.
I am really glad someone from the forum is getting some. I am anxious to see if they are A. armata or not. We will not know until they start to develop horns so you can count them. Most importers routinely call all MHDs A. armata due to lack of knowing any better and possibly from a lack of caring. They just want to sell them.
I'm glad you already have some experience with a younger MHD as that will help you manage these two. They look to be pretty new from their pics. Please let us know when you get them, I'm excited for you!
Oh, and on the group thing, THAT’S a GOOD QUESTION!! 
Let us know if you find the answer! 
Marcia
