Callistemons are fearfully diverse, and the growth rate will vary considerably with the species. Having said that, probably 4/5 of the species are pretty fast growers, going 2-3 feet a year in fertile soil. The main exception that I am familar with is C. viminalis 'Little John'--ironically, a dwarf form of one of the fastest! If they are being sold at 6 inches tall now, that may be the variety that they are, too. As for which ones will attract the native birds, those are likely to be the ones that are actually native to your region, or the species most similar to them.