do you intend to connect the X670 switches via layer 2 to the PE devices? If yes, then the X670 MLAG pair will not forward a frame it received via an MLAG port back through the same MLAG port. The MLAG port is a logical construct comprising physical ports of both MLAG pair switches. The ISC will not create a layer 2 loop for (correctly configured) MLAG ports.
You should use LACP for the multi chassis LAG to avoid cabling errors and ensure that both sides are configured for (multi chassis) LAG. You need to ensure that the PE devices can be used in an active/active mode, because all active MLAG ports will be used by the Extreme switches to send frames. An active/standby model of the PE boxes could in theory be implemented by deactivating the secondary PE's link via LACP. The possible solutions depend on the PE features.
If you want the Extreme switches to participate in VPLS/MPLS, you should not use an MLAG, but layer 3 connections.