I think stop stressing. Invite everyone and let them sort themselves out. People generally assume when they here 'baby shower' that it will be a relaxed fun alchole-free event. Don't stress about them mingling - they either will or they won't, but so long as they have at least one other person to talk to they will be fine. You may find that some of the people aren't 'into' baby showers so won't come, but others will and at events like this they expect to run into a mixture of people.

I must admit, I am a bit jealous: I would have loved a baby shower, but have never actually had one - so go and enjoy it for all of us out there who missed out!