Possibility of a 6 month mongrel moving in with us and eight yr old bossy border collie - advice please!

[Nancery]

For reasons I won't go into, there is a small chance we might have a six month old setter mix moving in. We have an eight yr old border collie, female, who is fine with him on walks (tolerates, but takes no silly puppy nonsense) but can be possessive over me and especially dogs that come into the house. Previous experience has shown that she is basically dominant, and a bit heavy handed with showing it, but I'm as sure as you can be that it wouldn't be taken to another level.
IF, and it's a big IF, we ended up with the puppy, how would you go about it moving in and things settling down...? (I do have ideas, and experience, but would appreciate more points of view and advice)
Thank you!

We had a year old BC when we introduced a young spaniel into the household. We had them meet up on neutral territory for a good long walk on the day we collected them we then brought them home together. We had to feed them separately to begin as the spaniel had been a stray and he would guard all food so he was fed in a crate and the collie was fed in her usual place. Over time, and as his training has improved, he has learned to sit and wait for his food and we can now feed them side-by-side. Sometimes they have a moment where the play gets a bit too boisterous but one usually lets the other know, they sulk in opposite corners for a while then get back together to conjure up more mischief.

At first they were very separate from each other but now they often sleep near each other, we have done away with his crate (it was pointless anyway, the collie could undo the crate door and used to let him out when she saw fit) and where one is now the other isn't far behind.