Ken is absolutely correct: SLC I/O can't be controlled by ControlLogix - this is done mostly by political reasons.
But they can co-exist on the same ControlNET network.
Let say: you have some PLC5,CLX,SLCs PLCs with mix 1747, 1771, 1794 and 1756 I/O on the same network.
PLC5 can control SLC, PLC5 and FLEX I/O
CLX can control PLC5, CLX and FLEX I/O
SLC can control SLC I/O and I am not sure about the rest.
My opinion: it is time to switch to 1756 I/O, once you use it you will never go back to SLC, PLC5 and possibly Flex modules.
Speaking of Flex: 1734 platform should replace it completely very soon.