Yes, you can do that, but Graphene is probably best used without Google apps at all.

That totally depends on your goals. For some it's security, for some it's privacy from Google, for some it's removing the ability from Google to (worst case) remote brick/block your device. The latter may sound a bit far fetched, but if I was employed by, say the ICJ, I would know what system I'd run to avoid that.

One of the things that I like about GrapheneOS is that it accommodates all these use cases and on a gradient (e.g. personally privacy from Google is also important, but I like that I can use sandboxed Google Play Services with most of its permissions removed).