I know there is some rule of thumb when it comes to sram and shimano ie if your running dual ring on sram its medium length but on shimano its short or something like that?

Im moving from a x9 back to shimano and thinking i need medium cage like i have already but the medium shimano is 10cm compared to 7.5cm sram. So am i thinking wrong?

According to the bumpf, 11-34 at the back and 3 rings up front will be too much for a Shimano medium-cage mech, and you should go for a long-cage mech instead. But I’m running 11-32 (I think) and 3 rings at the front with a medium-cage rear mech without problems. It’s doable, I think you just need to be a bit more careful when setting up chain length etc.

im running dual ring up front tho not triple

If you never cross-chain you can use whatever length mech you like. I like a short mech together with shorter chain, but big ring/big end of the cassette is not possible – you should never do that anyway.

A medium length GS cage would be ideal then GolfChick.

just seems weird to me that the shimano is 2.5cm longer than my current sram