1. Well, if money's going to the plushies instead of vital stuff, yes one can have too many plushies. Aside from that and space requirements, there's no real limit to plushies. [That reminds me... I need to send off the order for those plushies I've been eying...]
2. Yeah, it's "infantile." It's childish. Of course, I know that it's childish for me to curl up with my Gloworm every night, but I do it anyway. The little guy helps me relax after a stressful day. Childish and wrong aren't the same thing. If this guy likes his Care Bears, more power to him.
3. I see no reason to "grow up." I have plushies. I play video games. I act silly at work. I'm mature enough to know that life can be fun if you'll only let it be so, and that while there are times to be serious, there are also times to be silly.
And then there are times to scare yourself silly playing Resident Evil 4... Ah, good times, good times.