Hi! Welcome to GMing.
So. First - let me congratulate you on getting a good pool of options! That's usually the first hurdle. Choosing a functional group is the next, and when I'm doing it, I tend to look at the offered characters from the perspective of 'how well are these people going to mesh as a group?' and 'are they going to have a good blend of skills for whatever they're going to get into' - and around here, I usually like to try and have a fairly even blend of genders and preferences, because even when it's not the focus, around the E, it often comes up.
The E offers you the unprecedented advantage of checking up on people, so you can see a little bit of their posting habits and how they interact with people. This can be useful, but it probably won't save you from player attrition, so ... the people you can't take now? Find out how many are okay with going on a 'waiting list' - for the gaps that will eventually open up.
Personally, I don't like first-come, first served - I don't like it as a player, and I don't like it as a GM. Too much a risk you're going to get an unbalanced group, or something even less functional. But in the end, you go with what works for you.