Check the Spektrum receiver specifications it maybe that they output all 18 channels on the SRXL data link. You could then use the 12 channel Wingstabi (non-MLink) with 12 outputs and use the additional channels on the SXRL link to adjust gyro mode, gain etc.
All Wingstabi units have 16/18 inputs (depending on the transmitter/receiver link) regardless of the number of outputs. In the classic firmware the inputs and outputs are freely assignable. e.g. you could have aileron input on channel 1 and use any outputs on the Wingstabi to drive the aileron servos.