Some of us don't have big yards and pretty much have to use the dog park so our dogs can run, so I don't find the categorical "never" and alarmist tone all that helpful as global statements. It doesn't seem like much of a life if a dog never gets to play with other dogs. There's no living without some element of risk; you just have to try to be very vigilant and minimize it. Beth has had wonderful times playing with a couple of other medium to large, confident dogs.
HOWEVER, I would say: no, don't muzzle unless all are muzzled, only go at non-busy times, and I would NOT let my grey off with little dogs. You might want to wait until you know your grey a bit better, too.