How do I manage invitations and RSVPs?

I’m getting married in September, in New York where my fiance and I live. The wedding is out of state for both our families and a lot of my extended family will be flying in for the wedding. 

I have a large family and there’s about 180 people total on the guest list. My parents expect that the count is more like 120 (elderly, people who may not want to travel, people they feel like they had to invite and so on).

Our ceremony site is a place that’s very special to my fiance and me, at Yaddo, an artist’s community. It’s a small garden and the place of my dreams but the venue is very strict that only 100 people are allowed. I was fine with inviting the majority to the reception but not everyone to the ceremony.

My parents thought that we should invite everyone and in the event that we’re over the 100 allotted people (which we will be!) eat our words and tell some people that due to space they can come to the reception. We brainstormed a great idea – when we send out ‘Save the Dates’ to send a reply card with them, saying when the wedding is and the town and with some diplomatic, “because of space issues” without actually saying “because of space issues” asking them to  send an early reply… Formal invitation to follow. That would give us a much better idea of who actually plans on coming as opposed to guessing and hoping and then when we know how over we are I can make cuts where appropriate for the ceremony. Help – how do I do this?