This is the situation VLP currently run 7 car V/Locities (made up of 3 + 2 + 2) on Peak Geelong services only. IIRC platforms were extended at Nth Melbourne (5 & 6), Geelong, Sth Geelong and Marshall to handle 7 car trains . (Nth Geelong & Lara were already long enough).
With a view to operating longer VL consists VLP has trialled selective door opening on longer VL Consists to avoid platform lengthening . At this time there are no plans to run longer than 6 - 7 cars VL consists, rather in the first instance to run additional peak services once RRL opens.
The current order for 40 extra V/Locity cars will see all remaining 2 car sets built up to 3 cars, plus some extra 3 car sets. This arrives at a standardized fleet of 3 car consists which at least in the medium term will be operated as either 3 or 6 car consists.
With the RRL project IIRC the platforms at Southern Cross (15/16), and at Footscray (New 3 & 4), and Sunshine will be 220m long to allow for up to 9 cars in future if required.
The only line that really justifies 9 car operation is Geelong where on limited stop trains Lara is easily extended at the Up end, Nth Geelong at the Down end, Geelong at the Up end. Sth Geelong and Marshall would require expensive signalling and track work.
On the Bendigo and Ballarat lines platform lengthening would be required at many locations to operate even 6 car V/Locity consists at all stations . There has been talk of VLP proceeding with slective platform lengthenings at some bendigo line stations to accept 6 car consists .