Engagement Party Guests

You say only "some sources", but I can't imagine any reputable source saying that inviting someone to a pre-party but not the main event is polite. There would be no engagement party or celebration without a wedding, so asking people to celebrate you but excluding them from the wedding rings false.

That said, it's not literally against the law. Of course you can invite anyone you want. Just be prepared for them to be confused and/or hurt later when they don't receive an invitation to your wedding that they are (very reasonably) expecting.

Thanks for the response. That is what I assumed, but I suppose I was trying to avoid hurting feelings by excluding them from everything and also not wanting to offend my parents by telling them who they can and cannot invite, but can see how it may do the opposite and actually cause more harm than good. I appreciate the insight.

Its rude to invite people to celebrate an event that they're not actually invited to. I wouldn't do it.

All (reasonable) people know that there are limits to wedding guest lists because of budget and preferences, so I wouldn't worry about that. Might some people you don't invite be miffed? Sure, but that doesn't mean you've done anything wrong. Trying to fix presumed hurt feelings in advance by inviting them to other parties that aren't your wedding isn't the fix that it seems on the surface.

Also many engagement parties are thrown by parents well before guest lists are created and definitely before they're finalized. I think friends of your parents, especially those who you're close to, would be understanding of this. I would hope, anyway.