You don't need instruments to fly a Citabria!!!!!!
Whilst this is true, if I am
instructing, especially aeros, there is one instrument I do like to be able to see and that is the ASI, especially when looking at high speed UPs!
Thankfully the ASI is usually top left of the panel and fairly easy to see from the back, especially if you use a decent cushion.
As far as flying from the back seat goes, no reason not to apart from C of G limits or limits imposed by the person hiring out, I would suggest, if you are not used to flying from there you get checked out in that seat first.