The Forsaken would never use the Light to fight. They abhor it. It abandoned them in their darkest hour, in their direst need. Forsaken priests use their knowledge in the Light only very rarely to support the other troops, but they have no faith in it.
Gnome warriors - why not? Every race, no matter how small, can wield weapons and fight to defend its homeland - that's exactly how the gnomish infantry was formed to liberate Gnomeregan from the lepers.
Blood elf paladin - we spoke about that already. They got their power from torturing a Naaru and intend to use it against the Alliance, their stones back on their heads, who failed to protect them against the Scourge. To me it would make more sense if they resorted to the Light in their dark hours to give them hope, but if Blizzard had done that then they would have had to make blood elves part of the Alliance.
Troll mages really make no sense, honestly. I can't think of any justification for that, but they're quite popular among the Horde, so that makes me think they're just prone to magic. They can be great pyromancers or hydromancers, wielding those two elements without much effort. I also found
this archaeology piece and
this forum thread. It seems that Zandalari trolls actually knew magic quite well.