Because we want to do our part to help, we would like to get another rescue. Also, because we want a dog that can be very social I think we need to start it pretty young (less than a year?). Do you find this to be true?
Well... yes and no. And depends. How's that for an answer. :-)
If you get a young dog and really work on socialization, then it generally pays off in the end. But ultimate temperament is a 2 aspect thing - only one of which is socialization. The other is genetics. With a young dog, particularly one where we don't know the lineage, one would never know how well "tempered", genetically speaking, the dog is. You could do all the right kind of socialization and still end up with one that is slightly "edgy", fear aggressive, bad in strange environments, etc.
With an older dog, we already know what its basic "genetic temperament" is. Most rescues come in without a great degree of socialization. Some are terrible, but some turn out to be wonderful dogs, in spite of their socialization, not because of it. These are the ones that are coming from good genetically tempered lines. Any degree of socialization on top of that is just icing on the cake. Even the ones with bad temperaments can be turned around to make them bearable, though nothing can really replace good early socialization.
So there's two ways of looking at it and it depends on your perspective. I, personally, ALWAYS recommend an older dog (> 6 months), simply because we already know what the dog is going to be like. We know if it's going to be good with kids, cats, strangers, cars, how much herding instinct it will have, etc. With a young pup, we can tell somewhat but there are no guarantees. If your situation requires any sort of particular criteria, then an older dog will be your best and safest bet. Those without particular requirements - no kids, no cats, not a lot of strangers or strange dogs around, etc. - can often adapt themselves and their environment to fit their dog. If you have a kid, then the decision is more important. Since you're not going to get rid of your child (at least I hope not!) because of the dog, then you should be sure to get a dog that fits in to your environment.