Getting a second dog

I've had Mutley for about 8 months now and he's settled in reasonably well. On his walks he is extremely eager to play with other dogs so I thought it would be beneficial to get him a playmate. I've read on this site somewhere that it's not a good idea to get a second dog which is the same age as the first and I didn't really want another collie as two Mutleys would really wear me out.
But "the best laid plans ..........."
So I went to the rescue centre where I got Mutley from and test drove an 8 year old Black Lab and a 6 year old Border Collie. I did want to give an older dog a chance but the Lab was overweight and did not seem playful at all although he was very sweet. The Collie (male) was very affectionate with both me and Mutley and wanted to mount Mutley (also male).
So I ended up reserving a 3 year old Collie cross (Mutley's 2 and a bit) who got on with Mutley very well. He also seems a lot less excitable. I will be picking him up next Saturday with the proviso that if something major does not work out in the first week they will take him back and return my donation. I really hope it's all OK as returning him would be very hard.
Does anyone have any tips on what to do or what not to do when settling a second dog into the home.

Wish you the very best of look with the new edition to the family.

The best advice I could give is to provide both dogs lots of space and Intoduce them slowly.