My question was concerning if the rack board would handle a full length card or not. Also revealed is that it will have two slots, contrary to what is listed in the product description on their website. Below is the email they sent me. ( Posted with permission from Sonnet.)
Straight from the engineer that is designing the RackMac:
"Yes the Rack can fit a full length card. We do have a Red Rocket and are doing extensive testing with it in the Echo Express which uses the same main board. There will be a second PCIe slot but it will only be able to handle a half height, half length card."
Glad to hear this myself.