The mainline Electrostars aren't particularly suitable for metro operation. Part of the business case for the new trains will be higher capacity trains meaning fewer vehicles and crews are required to deliver the same level of overall capacity. Hopefully this will be allowing for future growth rather than locking in a reduction of vehicles, but it's unclear at this point.I wonder if it would make more sense to get new 120m trains for mainline services to the Kent Coast and backfill the Networkers with existing Electrostars.
120m trains in general don't fit well on the Southeastern network. Routes of out Victoria are mostly 160m for metro and Gillingham stoppers/Ashford via Maidstone. It's possible a metro order could work with a mix of 120m and 80m trains as a lot of the metro area is 240m capable with SDO aside from Victoria.