Reference: Supported NICs and Slot Configurations : Supported slots for SteelHead CX 6090 and 8090
  
Supported slots for SteelHead CX 6090 and 8090
SteelHead CX 6090 and 8090 supported NICs
Quantity
Supported slots
40GbE
2
Slots 2 and 4. These cards can be installed in slots 2 and 4.
10 GbE
4
Slots 1, 2, 4 and 5. These cards can be installed in slots 1, 2, 4, and 5 and cannot be installed in slots 3 and 6 due to hardware restrictions.
1 GbE
5
Slots 1, 2, 4, 5, and 6. These cards can be installed in slots 1, 2, 4, 5, and 6.
For optimal performance, it is recommended to alternate your installation of 10GbE or higher NICs, between slots 1, 2, and slots 3, 4, 5. For example, if your first 10GbE card is installed in slot 1, your second 10GbE card should be installed in slot 4 or 5.
Supported slots for SteelHead CX 4090
SteelHead CX 4090 supported NICs
Quantity
Supported slots
10 GbE
1
Slot 2. These cards can be installed in slot 2.
1 GbE
1
Slot 2. These cards can be installed in slot 2.