Why does the SX-100 use 6AU6 and 6BA6 mixers vs the common 6BE6...
It is not absolutely necessary to use a 6BE6 or similar mixer tube for mixing.
Looking at the schematic the V2 6AU6 is receiving amplified RF at its control grid from V1 while the cathode is being given oscillator injection generated by the V3 6C4. The plate current is a mixture of signals but the tuned plate circuit is only passing 1.650MHz.
At V6 the oscillator signal from V12 is injected at the cathode AND the suppressor grid. 1.650MHz 1st IF appears on V6's control grid. The plate current is a mixture of signals but the tuned plate circuit only passes 2nd IF 50.5kHz. V12 generates either 1600kHz or 1700kHz, depending on which sideband is selected.
A 6CB6 has an internal 280k plate resistance while a 6BZ6 has a 260k internal plate resistance. As an exercise for you look at the gain equations for a Pentode and determine why the 6BZ6 has a slight edge in gain over the 6CB6.
RL (external plate resistance) Assume 50 ohms
Rp (internal plate resistance) From Data sheet
gm From Data sheet
mu = gm * Rp = ?
Voltage gain = Av = (mu * RL) / (RL + Rp)