Woo! Questions. 

1. Yeah, it's pretty much just a crossover. But if magic is important to you when you're developing a character, throw it in. It could be mixed in with the plot pretty easy. I want a bit of randomness to the roleplay, and maybe it would explain the ferociousness of the virus and it's effects on the survivors. 

2. They're not a mystery, everyone knows about them. Most communities have at least one, and Barclay has several. They're generally respected (or feared).
Everyone knows the Supes got their powers around the time the virus struck, and that the Supes played a leading role in clearing the hordes in some way.

3. Barclay will have a population of a hundred or so people, and be quite remote, as with most surviving towns. It will look mostly like any other small town, with the exception of it's burned down buildings and swathes of empty buildings which are spraypainted to indicate they've been searched for the undead. 
The population that lives there is a mix of those who lived in the town before and those from outside - either way, they all live towards the centre of the town so that all lived-in buildings are surrounded by one large metallic fence. Buildings nearby the fences are used as watch towers, and armed appropriately. 
Everyone would have an assigned role by the Mayor of the town, and the Mayors decision is always final, even regarding decisions about other Supes. 

Other outposts are going to be up to you guys. You can have your character come from an outpost which is protected by a massive purple telekinetic bubble and full of ruthless redneck rapists or from a cult town which worships it's Supes and burns people at the stake. I don't really care. So long as you end up in Barclay or involved in the town in some way.