Well, if they were that severely underfed their growth could have been stunted. So they could be 4 to 5, although I'd tend more toward 2 or 3... but you never know, the most important thing is to make sure they get fed well now that you have them

As far as the ticks, it's probably just one of those weird things. One lucky tick just happened to find her I guess. As long as you cleaned her enclosure really well (ie empty out all of the old bedding, use a diluted bleach solution, etc) and you don't notice any more of them, then it's nothing to worry about.

Good for you for rescuing these guys, and just be sure to quarantine away from your other snakes (as I'm sure you already know)