RC systems were not allowed for any CL functions in contests prior to the appearance of the 2.4 MHz systems. One of the logical reasons for not allowing such systems was to not have any interference problems with other CL fliers as well as preventing interference with possible other "regular" RC systems in the area. If such systems were to be allowed in a CL Scale contest, it would be necessary to have some sort of frequency control process as in RC contests. With the advent of the 2.4 systems, such interference with other systems, CL and/or "regular" RC models, is no longer an issue when using those systems.
The CL Scale rules now allow "The use of 2.4 GHz radio control signals through the air ...are allowed for other than pitch control except for restrictions outlined in CL General Rules."
This rule allows the use of 2.4 GHz systems and nothing else.
Non 2.4 GHz systems are not allowed in AMA sanctioned CL Scale contests. Now, if you want to hook up any other RC system to your model for non-contest flying, there is nothing to stop you. Beware that you might encounter interference whenever you fly.
Keith