If Apollo is not drinking at all his eyes will be sunken and will appear small. He will act quiet and depressed. Easiest thing for the vet to do would be to give an injection of subcutaneous saline or similar if he is truly dehydrated.
I think if Apollo is active with the other budgies then he is ok and you should reassure yourself he is ok. Drinking is an activity that makes birds vulnerable to predation. Since Apollo is new in your home he would logically not make himself vulnerable in front of you. So unless he is sunken eyed and depressed, I think you could stop worrying.
And yes it’s fine for the other budgies to drink pedialyte: