What if you have only Horde Chars on Firemaw. Is it allowed to transfer a dead HC Alliance Char on Firemaw? Havent seen any restrictions? It would be the only option for me, cause its the only Cluster on Medium/High. Or is it Bug using? I heard it works tho.
Thanks.
Yes, I transferred hordes to Firemaw too while I had ally toons.
These are the articles for FCM/PCT
You can transfer to any of the realms within the same cluster:
Transfer overrides common realm rules. Not only you can transfer your Horde characters on Firemaw, but also as long as you will have at least one of them there you can create new Horde characters on Era Firemaw.
little bit weird. i would like to have them on ashbringer. dont know why you cant choose the other servers from this cluster. 
Yes on Firemaw most of the names are taken, it would be nice to be able to choose other servers from the cluster where the name is available.
Because Blizz wants all characters on only on server to each cluster - so they close down rest - is what I suspect.
Yes. The PvE to PvP restrictions on Era was lifted around new year when SOM transfers was implemented.