hmm, tricky.
i would say if you dont have any concerns about having such a small dog in your house (ie. things that can hurt them; furnature falling, small kids, other pets he/she might not get along with), then go for it! tyce was only 2.6 at 8 months so you never know, it might grow more...he's over a year and just went up again, but still only 4.2 lbs.
as for the health concerns, if you are careful with what you feed it, let it play with and the harness you use, as well as the daily precautions of the house and whatnot, there shouldnt be any problems that you can do anything about anyway. all animals and people can have problems that are unforseen, who knows maybe the 5 pounder would have problems too!
do it if youre comfortable with it!
