I have Ultegra BR-RS805 flat mount hydraulic brakes on my gravel bike, and Shimano RT86 6-bolt Ice Tech 160mm rotors.
There's a shim between the brakes and the frame to fit the 160mm rotors. When the guy at my LBS looked at my bike, he held the brakes and rocked it back and forth and said there was a lot of movement on the back wheel due to the shim and recommended that I use a 140mm rotor instead so I could remove the shim. As I have another set of wheels I'm building out for the bike, I thought it made sense to get a pair of 140mm rotors and then use those on the two rear wheels, with the 160mm rotors on the front.
As the RT86 rotor does not come in a 140mm size, would a 6-bolt 140mm SRAM Centerline rotor be compatible and a good option?