To the question, "Could Adam have Rightly stayed single (1Co 7) and not had children, without breaking this command?", first of all, 1Cor 7 did not yet exist. Second, if there were other members of humanity that were procreating, then it did not fall to Adam alone. Since, however, the narrative has Adam and Eve being the lone members of humanity, it's a given that they would have had to procreate in order for the command to be in effect.
To the question, "was Adam already 'married' when this command came about?" Dunno, and it's not important either way.