Definitely no special characters or numbers. I tried making vault characters for my played characters several times, and Ingo2, Ingo-b etc. are forbidden. So I've got Ingowardrobe. For my chrome dome Hobbit Rilibald I made a partially hairless Kindabald and an extremely hairy Notrilibald.
I've got Ingo on about ten servers. One is 100 and the rest are levels 5-7 except one who has been levelling via festival quests for years and made it to 15. Many of them will be sacrificed and the value of their goods transferred to a survivor. Going to be a bit of a waltz to get the 100 Ingo to another server if Elendilmir is closed.