Ok dont tell mickey we did this.....
once upon a time we had a ps for 4 (including one in a highchair). When my brother decided to join us we showed up and said our ps was for 4 plus a high chair. They made room without question.
i dont know how this would work with someone not in a highchair though. The problem is the 7th might change the table situation. Call and ask. if they already have you at 2 tables beause the biggest table is a 4top, then you might not have any trouble at all. call and ask and if they say it is a problem tell them you want to keep it and someone will stay home. then you can take your chances at meal time.