I wanted to add...not all probes are created equally. That temp probe you got that came with the kit will only work in a bus type plug, the pH (and ORP) probes are a standard BNC connection and the Salinity probe is a flat out custom plug that only works in the SL2. Since we only have 4 modules total we have to choose wisely.
I'm planning on running two PL4s, and an SL1 running 2 ph probes. That leaves me room for one more, although to be honest I'll probably upgrade to the RKE if I'm still in this hobby a year from now.