[@The 4 Winds]
1. Always

2. Yes, but you need triple approval from GM's to get it through. Also, the spots might be taken as Joshua said, though i thought we were currently suffering from a lack of Slayers, technically. At least, in Dragon Slayers, we only have Two, Jade and Ice. Then we have my Godslayer, another godslayer, another Devilslayer who has yet to make an appearance, and a Phoenix Slayer or two. best course of action, ask the GM's

3. If above is too much, then yes, Bone magic could be a thing, though I have a character with that kind of magic in tow, so I'm not sure if [@Caits] will be cool with it or not. If it's up to me, then yeah, you can haves the bone magic.

4. Yes, they're the main guilds, though if ya want to be bad, Shadow Heart is currently the only Dark Guild, I believe. Iron Enigma was a dark guild, but I think it's at elast appearing to walk the straight and narrow now.

Hope this helps.