Victron has said they will continue to make the previous generation batteries for some time. No timelines or commitments were made. In the USA, Victron currently has the 12V 200Ah NG and the 24V NG 100Ah and 200Ah in stock. If one of those three models works for you then go with the NG batteries. Or maybe wait until the others are released.
Decided to go for the NG system. Planing to start with 300Ah. In case i decide to add another one in parallel, would it have to be another 300Ah, or could i also add a 100Ah, or 200Ah.
You can mix different capacities in parallel. 300Ah at 12V isn’t a lot of battery capacity so don’t be surprised if you end up installing a second 300Ah