It could also have been because the Predators in AvP were younger. Hunting is a big thing here in South Africa and my friend told me how his father had killed an antelope that had a fawn, he wanted to take care of the antelope and protect it while his father just laughed and said the leopards would take care of it.
Scar doesn't have the father to chide him, maybe he thought Lex was a cute lil human and wanted to keep her safe, not fully appreciating the purpose of the hunt yet?
Then again, that's still thinking in human terms. I'd much prefer that getting the respect of a Predator was no different than painting yourself with a target and walking into a psychotic gun nut's shooting gallery.
As for the topic of multiple Predators, I'm pretty sure that, going by the name, one Predator will be the focus, but wasn't it mentioned somewhere that this was going to deal with the lore? Maybe the other Preds are for a homeplanet scene?