Personally, I think that you should only recruit a GM once you know that you need one (meaning, after the RP has already started). 

Assigning a co-GM before the RP even gets off the ground can be tough, especially if you don't know the co-GM in question super well, because they haven't had the chance to establish much dedication to the RP yet. Nobody has. Also, you have to teach them everything they need to know about running it from scratch.

Generally, when I feel like an RP is getting a little tough to manage and I want another thinking mind to help me organize it, I'll pick someone out from the crowd I already have. That way, I'm able to single out someone that I feel has already proven themselves to be dedicated to the RP, and they already know a fair amount about the RP due to having participated in it for so long. After that, you don't need to teach them as much. 

In the meantime, don't worry about recruiting a co-GM you may or may not need. At worst, having two minds where you only need one could potentially overcomplicate things. Best thing to do is just start the RP on your own and if it works out fine that way, then great. If it proves to be a little much and you want a co-GM, go get yourself one.