Crests can be used with the following kept in mind:
You cannot put the crest of someone else's family on your item.
You cannot use a crest in a way to make an item seem "official" issue from a royal family.
If you want to use a family crest that does not exist, the crest must be described in the LOOK of the item.
Guild Crests
In no particular order, really. Have Rathan families mixed with Elothean Houses and who knows what all else mixed in here.