Adults are allowed to tent as they wish - alone or with others. Youth are 'strongly encouraged' to tent with other youth. The following is a cut and paste from the 2012 Guidebook to Adventure, "If you choose to provide your own tent it must be a two person tent and fee of all food smells." With that said, in 2010 I took a 1 person tent as the only adult female in a crew... no questions asked, this year we had 3 adult men in our crew tent individually, 2 in 1 men tents and 1 in a 2 man tent.... no questions asked, BUT the common thread here is that all the solo tenters I mentioned were adults!!
Figure this out BEFORE you get to Philmont, the Rangers have a lot that they are required to cover with each crew and how your crew does 'tent partners' is not part of their thing. I'd suggest finding a lightweight 3 man tent and taking it or going low key with the oldest youth as a solo tenter (especially if they are close to being 18yo). [As a note, we took all of our own tents and the Ranger never inspected them until we were at our 1st camp, only asked if we had tents.... but your Ranger may vary!]
If you don't have your tenting figured out before you get to Philmont your Ranger will be more likely to question ALL other aspects of your preparation.... we always try to make our time with the Ranger as simple and easy as possible.... jump through their hoops and they are on their way in a day and a half... otherwise your crew is 'red flagged' and you will continually have staff 'checking on you'!!!