We use a lot of Brogdon seedlings, just because we have a very productive tree, so we get a lot of seeds. They work fine.
Realize that unlike citrus, where the rootstock affects the cold-hardiness of the scion, that's not the case with avocado. So the only concern about a less-cold-hardy rootstock than your scion would be that the rootstock could freeze out from under the scion. If you graft low, and cover up above the union with mulch on your coldest nights, that also ceases to be a problem.
Malcolm Manners
Horticultural Science
Florida Southern College
Lakeland