If the amp has 7.1 speakers outputs (ie, 7 speakers teminals (front L+R, rear L+R, Surround back L+R, and centre) and sub pre out) and does 7.1 decoding then you'll be fine....
But if the amp only has outputs for 5 speakers and a sub, and only does 5.1 decoding, then there's little point getting another pair of speakers as they will have nothing to connect to.....