I also like that someone can only have 10 gremlins running around at a time. With this system, there's no need for people sharing them back and forth. I just drop them in whatever town I see at this point, especially since it nets me at least 2 goo each time I send one out.
And as for the ghosts, you can pop 5 of them in each town you visit. I've noticed though that you can get 6 or 7 if you act quickly. As you're down to one last ghost, if you move quickly, you can get any others that are close by on the screen. I also don't think it effects how many are left after you leave. Any time you show up in someone else's town, there are a ton of ghost.