Does anyone know what regulation or policy covers paralegals (27D) are not suppose to have roommates in the barracks?

It could be that you can't find it because it's not a thing.
Your friend may have just convinced a commander that it would be a good idea, or there may have been a local command or installation policy for that.
And it probably is a good idea if the space in the barracks can allow for it.

But think about it this way, as a 27D you have access to sensitive but not classified information.
There are Soldiers with a TS/SCI with access to classified information that no one else has access to or a need to know, and those Soldiers also have roommates in the barracks, because there is simply no regulatory requirement for a single barracks room based on MOS or access to information.
That's why we sign NDAs, we expect Soldiers to not disclose that information, even if they have to live with a roommate.

Nope - Not a thing. As a paralegal you should be able to research that bit yourself. The reason you can't find it is there is no such regulation. I commend you for asking here in RP though.