There is a difference in declining an inquiry and declining a request to book. You don't need to "pre-approve" or "decline" and inquiry. However you must "accept" or "decline" a request to book, and do so within a certain time frame, or it will affect your stats.
It's not unreasonable to "decline" a request to book if the party doesn't appear to meet your criteria. If the party wants to book your place after you have already declined them, they can indeed send in a new request to book with the new information they provide. Tell the guest to resubmit their request, showing the proper number of guests.
It's important to communicate clearly your expectations in your listing and by messaging on the Airbnb platform. Don't make any promises or agreements with the guest without backing it up in writing on the Airbnb platform. If you do, then Airbnb support will be more limited in what they can do for you if things go wrong.