Digital is preferred on the tail and on cyclic there's no need for digital. However, you don't have to have digital on the tail. Analog will work fine. Make a spacer out of wood or something if the servo tray is too big.

I can tell you from personal experience that there is a big difference between using a $4 servo and a S9257 on the tail. However people that use $20 or $30 servos on the tail say they work great. If you want a cheap option some people at my local clubs like using an esky digital servo on the tail, I'm not very fond of them (lots of times 1 out of every 2 or 3 don't work brand new) but it might be worth a shot. Keep in note that most servos including the esky digitals will require a homemade spacer. Maybe there are smaller servo mounts out there but I keep mentioning a homemade spacer because I just saw one last week for the exact purpose.

One last thing, if you use an analog servo, which will work fine, be sure to read over your gyros manual, sometimes there's a switch for digital or analog servo, and I can't quite remember but I think you don't use head hold when using an analog servo (don't quote me, I'm not too sure on this).