1.) More than one character is allowed, but capping it at two for now (suggestions always welcomed).
2a.) Whatever the rp calls for and best fits the group. For now, I'm waiting to see if the rp keeps up and then the details can be hammered out as we go and works for our characters.
2b.) As for the world's reaction, I'll leave that up to everyone else. For now, I'd suspect the government would want to get a handle on things and remain somewhat benevolent toward the supernatural community, but also I'd suspect (like other political issues) it'd vary per state/country, giving us freedom to explore different scenarios with our characters.
3.) More so early days of X-Men, but with some government foreknowledge or suspicion beforehand (so, more top-grade, secretive agencies within the government, but only those with clearance to work on top secret cases). Some characters may have been exposed prior to the revealing in the opening scene, but in general, no, the knowledge wouldn't have been widely known (or at least accepted).
(The above can be altered in certain cases and am open to whatever suggestions others may have.)
4.) The subtlety your character chooses to exercise within the scope of their power is completely up to you. Some powers may be considered sub-powers, but could later lead to greater advances and growth as the character learns to explore their power. Others could opt for weaker power and play more of a humanistic character. It's whatever writers feel comfortable with. The only rule of thumb for this, is the restriction of god-modding. As long as your character isn't ridiculously overpowered, I'm okay with just about anything.