Again, on a nice day, you could try mixing them now for short periods of time. The trouble with your hand rearers is that they don't have a mum to protect them, and if your other goats are the goatlings, they will probably be desperate to beat up the kids (no matter how nice natured they are, it's intinctive to want to beat up the ones inferior to you). Having said that, the kids will be agile enough to keep out of the way.
I would probably leave it until the are 3-4 months as you say. We normally leave ours until the kids go down to only getting 2 feeds of milk a day. No sesne in making extra work by having to extract kids from a big field with adult goats getting in the way just for you to give them their lunch! It also helps it they know not to expect lunch, when they are out for the day grazing, as then they won't hang around the gate looking for you and for lunch. Teachs to go off and spend the day out in the field with their herd.
Beth