I personally believe there's a very strong ridership argument for Hamilton and Niagara, however anything inbetween like Grimsby or St. Catharines would probably be some form of loss.
Obviously it's hard to extrapolate data and determine ridership, but as of 2023 it looks like this
View attachment 720165
Hamilton ridership is strong and so is Niagara, especially given the frequency, however St. Catharines very clearly lags behind. Given the horrible frequencies, I'm not too surprised.
As for the size of the consist, it naturally depends on frequency/period, but assuming little change from now, Off-peak 6 car trains would be more than enough already, but 12 car is probably needed in rush hour to handle passengers from Aldershot to Union.
The closest analogue I can see is the Kitchener Line which saw massive frequency boosts from the 2017 data I have to 2023, and I'd estimate hourly frequency would net ~50% boost in ridership. But this is some pretty flimsy extrapolation.