Can you configure different MTU for subinterfaces under the same main interface? How do 7500/GSR/ESR routers behave in this scenario?

You can configure different IP MTU with the ip mtucommand on different subinterfaces. When you change the MTU on a subinterface, the router checks the MTU from the main interface. If the main interface MTU is set to a lower value than the one configured on the subinterface, the router changes the MTU on the main interface to match with the subinterface. Thus, the physical MTU configured with the mtucommand on the main interface needs to be higher than the IP MTU configured on the sub interfaces.

Packet memory is carved based on the highest MTU configured on 75000/GSR. There is one exception to this; the Engine 4+ linecard does not require to carve buffers on MTU change. On ESR, the packet memory is carved at boot time and is not affected by MTU settings. So if you change the MTU, you should not have any impact on ESR.