If you both have your guidelines and trust each other to stay with the guidelines and not become attached, i dont see why you shouldnt but your aware of the risks.
you only really need to know if you trust yourself and your gf enough to allow this person in your relationship so early on. Also she knows you dont want 2 guys on her are you secure enough that she wont say to you after this that she trusted you with another girl but why cant you trust her with you and another guy.
It may not seem important now but it is one of the risks you face after you do 2 girls.
A mistake is always forgivable, rarely excusable and always unacceptable.
Robert Fripp