You will only need a tail motor ESC if the heli has a tail motor, obviously.
For the tail motor ESC, two characteristics are important:
High throttle resolution (256 steps or more)
Many airplane ESCs have very low throttle resolution (typically 32 steps) because airplane radios typically have a ratchet on the throttle stick which limits the throttle resolution anyway.
If a heading hold gyro is used with an ESC with low throttle resolution, the tail will "wag" as the gyro tries to find the correct throttle position to hold the tail still.
If a yaw rate gyro is used with an ESC with low throttle resolution then the tail will creep left or right because you will be unable to set the tail throttle to the perfect value with the revo mix.
The GWS ICS-50E and probably other GWS ESCs are known to have only 16 steps of throttle resolution
High switching rate (>50 khz) (for brushed motors)
Many micro helis use metal brushed tail motors. Theses are easily recognizable because there are two types:
The popular "N20" style motors all use metal brushes.
The IPS-style motors with grey endbells use metal brushes.
These metal brushes are very fragile and are easily destroyed by the high current surges generated by low switching rate ESCs. Therefore it is desirable to use a high switching frequency ESC to extend the operating life of the expensive tail motors.
The IPS-style motors with black endbells use carbon brushes which are less fragile, but they will still last longer with a high switching frequency ESC. IPS motors (both grey and black endbell) are rated for about 2 amps max current.
The GWS ESCs are described as "high frequency" but they're only 2.8 khz. It's high frequency compared to the 50 hz control signal but it is not high frequency compared to the the TREC and other high frequency ESCs.
Some people are using the Feigao 1208436L brushless motor + Phoenix 10 ESC for tail control, but this combination has wagging problems and does not work as well as a standard brushed motor + high frequency ESC. See also the Section 23.6, “How gyros work” section for more info on tail motor ESCs and mixing options.
The following ESCs are usable as tail motor ESCs
JMP HF100 (brushed 100khz switching, 256 steps, 1.5 amp)
Schulze Slim-105He (brushed, 100 khz switching, ? steps, 5 amp)
Dionysus Design TREC ESC (brushed, 100khz switching, 256 steps, 5 amp)
Castle Creations Phoenix 10 (brushless, 10 amp, ? steps)