Mellanox connectX-4 adapters should work properly in any server, we don't have any known limitations.
In case you are having issues to work as expected with a specific server, please do not hesitate to open a support ticket by sending an email with all the details to email@example.com.
When choosing a server for 100G, think muscles
Recent CPU. Enough and fast memory DIMs. X16 pcie gen-3 bus. SRiOV support (in case you go with cirtualisation).
Do you have a reference for a model tested w/ more than 6 PCI slots ?
As already mentioned, Mellanox ConnectX-4 100G NIC works in any server that has a PCIE Gen3 x 16 slot. Also depending on the applications you plan to run you may need some RAM and CPU to match your requirements.
If you need more slots for other purposes you'll want to talk to server vendors and integrators. They have system engineers that can advise.
If I were in your shoes, I would go to my server reseller or integrator and ask for assistance.