you had better rid that s75 off your trex before it causes you to crash. those things are for park fliers not handle the duty cycle, stresses, and extremes of being on a 3d helicopter like the trex. ESPECIALLY as a tail servo probably has the hardest job.

common tail servos are:

analog-
hitec HS50 - small/lightweight, weak, fast
hitec HS56 - small/lightweight, karbonite gears, fast, very durable. probably one of the most common & recommended servo for the tail
hitec HS81 - uncommon, strong torque, hyper fast, slight weight penalty, slightly bigger than the submicro servos, inexpensive. i use this on my own heli as an experiment and i ended up liking it. i was originally going to do the hs56, but im happy that im going to leave it.
futaba S31-3,7,8,10 - cheap, futaba, nylon gearing, it works okay i guess.

digital (gy401 or digi gyro needed) -
577/877 - new class of cheap & sub micro digi servos. who knows about geartrain support???
futaba 9650 - ultimate servo for the trex tail. $60 each. a very big & heavy servo. will probably affect CG.
JR 281? - some pros use these for everything tail or cyclic.