319s / 321s should be confined to the Tring stoppers only.
Looking at the current LM timetable out of Euston, it looks perfectly logical to cover both the Tring and MKC services using 319s (presumably these are currently 350/2s plus a couple of peak extra 321s). Typical journey times of less than 45 mins and around an hour respectively - compatible with the majority of current journeys on Thameslink 319s, I'd have thought.
Off-peak, it looks like 7 diagrams can cover those services, with an extra 3 or 4 diagrams at peak times. 7x 12car formations, 3x 8-car plus a single for the St Albans branch could be covered by a fleet of 30 units, for example.
Out of all these swaps being discussed, I'd say the only justifiable complaint from passengers will be those using the Tring and MKC services, who would see a downgrade from an AC 350 to non-AC 319 on most services, although this could be tempered somewhat with the provision of more 12-car sets vice 8-car.