Thalyssra's spot on reason for why the Nightborne are joining the Horde.

[Jakerel]

What does she say?

Well it's not word for word, but the opening VO line is about inspiring them to resist, but the closing VO line, and this isn't quote for quote now, much more along the likes of "the more of the Shal'dorei that stand up and fight, the less of MY forces that might be lost in the coming fight"

Lady Liadrin's VO lines for that quest are very much about getting word out that help is coming, and showing the Nightborne that there is hope and a future beyond the tyranny of Elisande. Tyrande's concern is only about her self and her own forces, she cares very little about the end result for the Nightborne, she just wants the legion defeated.
And I get that, I really do. But from a writers PoV, the whole thing seems very coincidental. They either looked at it and saw a opening in the writing to explain why the Nightborne went to the Horde, or it was the goal from the start that the Nightborne would ally themselves alongside the Horde after the end of the legion and thus the story was written that way from the get go.

[dd614]

Remain Nightborne!

[CreatureLives]

but they all have to conform to human standards

I don't think they have to conform at all. They are all just like minded people. The Void Elves are going to be a very interesting addition because they are the first ones that seem different to everyone else (I guess the Dark Iron could maybe be thrown in here too). Obviously Draenei, Humans and Night Elves have different cultures but they have a lot in common in what they believe in and want out of life. That's why the Alliance has always been so strong. They all trust each other and have each others backs. Nobody in the Alliance is a slave. Nobody is forced to do anything. Yeah, maybe you won't be let in or possibly kicked out if you are trouble but nobody is saying you have to behave any way. That's just how everyone wants to be.

Also: Humans/Gilneans, Dwarves and Gnomes aren't all that different from each other.