There is absolutely NOTHING wrong with using the extra 4 wires for another LAN, in fact it is in the CAT5 standard. What is not supported is using them for phone. Ringing current with old style indictive ringers may cause noise on your data.
In fact, if you only have a single network, there can't be any crosstalk since you are really using a bus and all the cables will have the same data on them anyway. The only reason you have hubs is to maintain the topology but everyone is still talking one at a time. Crosstalk that does exist is between the read and write lines.
The different twist rates are still adequate insurance against crosstalk, even if you do have 2 networks running.
Most of your crosstalk will happen in the terminations anyway. (NEXT Near End Cross Talk)
Be sure you maintain the twist all the way into the keystones and keep the wires as short as possible.
