Terrispeople haven't always lived in the region they live in now, correct? TLR built his citadel where there were mountains, but turned them into plains. Apparently, Luthadel was difficult enough to reach pre-TLR that several people in the expedition died before arrival. Of course, some of them were killed, but to get back on topic... I think TLR forced them all to move up north when he took power. That's what I'm going to do to my family.
People can tell the difference between the nobles, Skaa, and Terrismen by sight. Not all Skaa can pass as nobles, but some can (just those with noble blood?). I don't remember seeing anybody mistake a Terrisman for anything else. Then again, they all dress funny and stuff. Just cast people that look similar enough for each race and it doesn't matter what was intended?
I have to stick to the Tennant-Jackman Kelsier-Marsh combo I suggested a while back.
OR!:
Kelsier will be played by Rand al'Thor (he already has stuff on his arms!).
VinGT will be played by Megatron (boyish figure and good looks!).
TLR will be played by the Ringworld.
Spook will be played by Dolly the Sheep.
Sazed will be played by a ball of yarn.