I got an OMM Cold Springs (disc) rear rack... I use it on my HT. I'm not sure what you mean by "compressing" your cassette and disc brake caliper. I know that the QR mechanism is supposed to compress the hub a bit when it's locked-down (this is why you build a QR hub a touch loose) but I don't know how it would compress your cassette.
I know that when I ordered my OMM rack, I had to specify "DISC" on my order. Are you sure you got a DISC rack? Call or email OMM, Channing is a good guy and he'll help you with whatever problem you've got.
On a side note: I'm not sure what those springs on the QR skewer do exactly but I transferred mine over from my old ones when I mounted the Cold Springs. Good luck!