I need to make a wedding cake that will feed at least 75 people but Im not sure which tiers size to use. 12, 10, and 8? will it look ok?

That will feed 118, but will look fine. 10, 8, 6 will feed 74. 12, 9, 6 will feed 100.

No, having an 8" top tier will look silly and massive.

5-8-11=73
4-8-12 = 80
4/6/8/10=70
6/7/8/9=75
5/6/7/8/9=83

.............8" top tier will look silly and massive........

It depends on the rest of the cake really. I have done a few w/an 8" top and sure don't think it looked 'silly and massive'.
But then I don't like most cakes that are 4,6,8,10 either

Ideally there should be 4" between the tiers ie" 16, 12, 10, 6.

To serve 75 people I would consider any of the following:
12x4 and 8x4" rounds
12x4; 8x4 and 5x3" rounds
12x4; 9x4 and 6x3" hearts
9x13x4" (sheet) and 6x3" heart
9x13; 10x7 and 7x5 ovals w/a 4 or 5x3" round
12, 9, & 6" hex
10" sq; 9" heart w/4 or 5" round (place the heart point on a sq corner/point to point)
10" sq w/ 6" round
9x13x4 w/6x3" round

I don't agree with the "ideally" part. What sort of design will this have? Some look better with much closer spacing, some look better with 3 inches, it all depends.

Well she wants diamond shape cake and the middle tier will have a diamond pattern on it. So Im trying to figure out what to do with it.