First, I know (or at least I think I know?) that there's no etiquette requiring me to invite coworkers. It's honestly easier for me if I don't, because as I've mentioned on these boards before our guest list is crunched.
The coworker situation has been awkward because we (a) have a very friendly office, particularly my individual department; (b) people have been extremely inquisitive about the wedding, esp because some folks knew my FI when he was a student here and we are getting married here, at our alma mater -- I know you are supposed to avoid discussing the wedding with anyone you won't invite, but no matter how many times I use the 'we're having a really small wedding' line that doesn't seem to shut down conversation, and being evasive seems to make people uncomfortable, and (c) I had a very close coworker, almost-friend, who got married a few years ago and did invite our department (8 people including spouses). So I've been proceeding with the idea that my immediate group of coworkers might have to take precedence over some friends, which sucks, but FI and I are having to make a lot of concessions to others' expectations, and are used to this.
So now, to add complication, said close coworker recently left to take another position, and we are working on hiring a replacement. Our tentative timeline puts the replacement in this job somewhere 1-3 months before invites go out. So now I'm not sure: do I just take this excuse to skip coworkers entirely? (can I do that without being rude?) To me it seems rude to invite someone I've known for such a short time (surely this will come across as gift-grabbing?) but since this person will be my office mate, I don't see how I could possibly invite other coworkers but not them. The only coworker I really have emotional investment in having present is my boss, but I would not invite her unless I invited the whole group. Really not sure what to do on this.