I just went to the dog park with my 3.
At the gate was a lady with an Airedale, she grabbed the dog so I could get in and it was lunging and growling.
She assured me that he was friendly, just over excited and wanted to play.
I was very apprehensive about taking my dogs in but as soon as we stepped through the gate (my dogs were still leashed at this point) the Airedales body language changed straight away, it bowed its front legs down and its tail was wagging like crazy, after that the lady released him.
Sometimes it does pay to listen to the owner, they know their dog... but still, it pays to be cautious, there are a lot of idiots out there.
I let my dogs off leash and they all played happily.
Re: Picking small dogs up.
I always pick my Crested up. He's a 5kg weakling and fearful of other dogs.
The only dogs I want him associating with are dogs I KNOW are friendly and not pushy so I can improve his confidence.
If someone has other small dogs at the park I still do the same... wait and asses how they are reacting to the other dogs before putting mine down.