Do 14x7 ralley wheels take the same trim ring as 14X6's? Or is it 7's and 8's that are the same?

No, but 14x6 rings (2 1/4" deep) will fit on 14x7 rims, but you see too much of the gap. 14x7 rings are 2 3/4" deep (same as 5 spoke rims).

There is no 14x8 GM rim