Yes, you can add and remove runtimes. If you only have a few, there's no need. Keep them all loaded. If you have dozens, you probably don't want them all loaded at once.

I don't add and remove runtimes every time I use Poser. Usually, I work on one project (or one type of project) for several days or even weeks at a time. So I'll swap out runtimes when I start a new project (or get a new figure to play with).

I have a large external hard drive - lots of space, but kind of slow, since it's external. I keep most of my runtimes there, but will move one or two to a faster drive when I'm working a lot in them.