Do you have a source for either of these?The Rivian is a 400 V truck and the CT will be too.
AFAIK, neither Rivian nor Tesla has announced the voltage architecture on these products. Rivian filed far a patent for series/parallel, but that was quite some time ago and it may or may not be implemented (lots of patents are never brought to market).
800 - 900V has significant advantages in both charging and system architecture. 350 kW at 400V requires a significantly beefier cable than one at double the voltage. Internal wiring on the vehicle also can be smaller/lighter/cheaper. Same goes for motors - higher voltages have advantages in winding wires size, weight, cost, etc. (look at Lucid as an example).
Every EA location I have seen or pulled up on Plugshare has both 350 and 150 kW chargers, but it is possible a few may have only 150kW.
Yes, RJ is ultra conservative in his promises and having it clear that 300 kW is dependent on SOC is a detail most companies gloss over. Tesla, for example, touts the Model 3 LR can charge at 250 kW, but that only occurs at a SOC of between about 10 - 16%. After that the taper kicks in pretty significantly.Note that times for either option will be longer than that because of taper. I thought it interesting the amount of emphasis RJ put on that aspect of it in the video interview.
We don't yet know any real charging specs on the Rivian, but RJ has stated that the battery will be "smart" an adjust to charging and usage patterns of individual owners. I'm very interested to find out what parameters change.