That is not exactly true it would appear to be a lot more complicated.
In cata the Val'kyr appeared to only be able to raise humans, and the only way the Forsaken were able to raise the dead was via the Val'kyr. So as far as Forsaken are concerned only humans can become Forsaken.
I believe at one point either though one of their short stories or in game, it was pointed out that the Lich King was and exceptionally powerful being when it came to created undead so he could created other types of undead. Obviously, Sylvanas herself is proof of that. However, to the best of my knowledge at this point in time only humans can become intelligent undead.
I want an undead durid naow !!!!!!
I don't think we know that. Unless it has been explained elsewhere, all we know about Arthas transformation into a death knight, that we see him running off into the cold by himself and then next he is a death knight. Although at one point he cut out his own heart so he much have been dead at that point.
Spike Flail - US Mal'Ganis | Currently 11/11 M | Art by ElyPop
On Death Knights: Blizzard specifically stated that Death Knights who pick an "old" alive skin, are not dead. The ones that pick the sickly, dead looking skins are dead.
Ok I asked it the wrong way and allow me to rephrase.
If sylvanas is bringing her Val'kyrs to SoO to expand UD population which Garrosh forbade (granted this is her only chance since next warchief/council will turn her down for sure), should she use dead humans from ally force to maximize the outcome? Or it doesn't matter which race the Val'kyrs will use because they will all turn to the current forsaken human-like undead?
Below is a line from the in game quest Lessons in Fear:
Our enemies are becoming smarter - adapting to our tactics. Recently an attack was made upon Pyrewood Village, southwest of here. Our troops occupying the town were decimated, of course, but what made the attack especially interesting was the racial composition of the attacking force. No humans were used.
The implication is that humans are the only ones susceptible. Using the words racial composition implies multiple races non of which were human. I stand by the statement that only humans can be raised by the Val'kyr.
Naaa, i don't think she will be killed, at least not in the same way of Garrosh, if she's killed she will be killed in a Horde vs Alliance conflict, but it seens that Sylvanas try to ressurect the Garrosh army to fight with her but Lorthemar stop her, and it seens that she ask if she can ressurect the Alliance soldier and Lorthemar say that's her and the Alliance problem (So... that's a Yes do whatever you want with them)