So we have a 6 year old lab/collie mix named Shadow whom we've had since he was a runt. A friend of ours wanted to give their new 4 month year old a new home with a bigger yard to run, so we volunteered. I've never owned 2 dogs before and it's been interesting. Obviously it is a big adjustment for both dogs, but does anyone have advice or some dos and don'ts? Right now, the little one just tries to sniff the big one's face to play, but he doesn't necessarily like it and barks aggressively to try and back her off a bit. Clearly all she wants to do is play but I'm not quite sure the big guy understands. I'm assuming these 2 just need some time to set boundaries and get to know each other? He hasn't tried to hurt her, despite showing teeth here and there, his hair never stands up on his back and his tail is always in the air wagging, so I'm assuming that he doesn't hate her. Is there anything I could do to ease this process?

Correct, Shadow is the 6 year old male and Rosie is the pup. I think they took a big step today; Shadow barked her down on her side and she stayed there, which is a great sign of submission. Since then, both have been very calm. She will be fixed at the end of the month. The signs are looking good I think. They even drink out of the same water dish without issue. He really doesn't seem to mind her when she's not messing with him.

Good to hear. All of the opinions I've asked for about the dogs eventually living well together have all said that their behavior is normal and things will be much better in a week or two. This whole experience is wearing me out, frankly.

Its actually better to have 2, The pup will learn behaviour from the other 1 ( where to pee and commands like sit ect ) Walkin them can be awkward but just be calm and patient and they will be fine.

You will find the older dog will give the little 1 into trouble a lot, Just make sure he just gives into trouble and doesnt try to dominate.