I know a recent thread has hashed the general point about single vs part of a couple to death. I'm not referring to splitting known couples, but whether it's acceptable to "categorize" truly single guests and determine whether or not to offer plus ones.
For example, the relatively young cousin who is very single and rarely dates, and who will be coming with her mom and brothers, vs the older friends of the groom who probably would like to bring a buddy or casual date. Can I extend plus ones to the non-family guests who are traveling without worrying about adding them for every single "single" person we know? or is this a one-size fits all rule -- all get plus ones, or nobody does?