"The total length of each MII trace should be less than 6 inches, or 6000 mils. The traces should be length matched within 20 mils for 1G transmissions and length-matched within 50 mils for 100M or 10M transmissions. RX traces must be length-matched to the other RX traces, and TX traces must be length-matched to the other TX traces. The number of vias and stubs on the MII traces should be kept to a minimum."
Magnetic Isolation No metal should be under the magnetics on any layer. If metal is needed under the magnetics, it must be separated by a ground plane at the least. Metal under the RJ45 connector with integrated magnetic is allowed.
Source Texas Instruments:
Chassis / GND for RJ45 with Integrated Magnetics 
Source NXP APP Note 4215:
That's it !
