S: These signs often mean the dog is aroused at a minimum, it sounds like the pup is not confident at this point.What is your intepretation of why they are doing this? Body language is excited, sometimes hackles up, some tail wagging.
S:I guess the real problem is that I would be trying to give you advice based off your diagnosis of the mind state of the pup and the other dogs at the time, which may nit be as I would see it.I have been correcting it by saying firmly 'quiet' or 'ah ah' and if it persists, putting the pup on lead and walking it away from play and returning in about 30 seconds when they are quiet. This was working, but now the barking just keeps happening on and off the whole time. Fortunately, they are playing with even tempered dogs who just ignore the barking.
If the pup is anxious, using pressure, even as little as a firm tone may not be the way to go.
The problem may be in the play style the your pup has, which might mean that your pup expects the older dogs its now playing with to play like your pups old litter mates. This needs to be adjusted so that the pups expectations are not missaligned.
I might with the above body language points spend more controlled time with your pup and other dogs rather than just uncontrolled play. It seems that the pup may not yet have the ideal socialisation value of the other dogs and is insecure at these times of play.
This is a concern as it often translates to fear aggression when the pup develops a little maturity.