rotary encoder versus glass scales

I am in the process up gathering info and parts to upgrade an old
Hurco Bedmill to PC controls.I have ripped all the old electronics
off the machine cept motors and limit switchs ..I have new servo
drivers in hand and just gotta get a coat of frsh paint on the
machine and will be ready to wire something up....Cleaning on the
machine today I found that the X-axis motor's rotary encoder had been
disconnected and somebody had installed a glass scale on backside of
the bed for that axis.Reckon I should try to find a new rotary
encoder or is the linear scale gonna be alright? I been reading about
rotary optical encoders but havent a clue at the electrical
differences between them and the linear scales..I want to keep this
simple where I can and save the sweat for the worthy
problems..Hehhehehheh
Then another question if somebody wants to take a stab at it is
why did the old controller make use of a tach signal from the
servomotor and these PC run conversions dont use that signal?--Or
maybe they do and I havent got to that part of the reading
materials..This is my winter project and I got a lot to learn yet.I
figure this isnt the last time I'll ask questions here.
Thanks
Steven