i2600 hardwired serial fail-over cable

As far as I know the failover cable has a special pinout. As long as the CAT6 cable you use has the correct internal wiring I don't see any issue, but making your own network cables is to me a big no-no. It's not easy to achieve the same level of quality as the cables you buy. Hardware failover is just a voltage "sent" though the cable and if the voltage drops it triggers a failover, you don't have to wait for missing UDP packets. BUT, there are restrictions, like maximum length of 15 meters and it has to be a direct connection. Also, Hardware failover is rarely needed, network failover is easily fast enough for all installations I have done so far. Are you sure you really need it?

Check out https://support.f5.com/csp/article/K1426