Hey there!
I’m using a grow shedule posted on the ILGM website. This one recommends to flush just before you switch to the new fertilizer so I’d do that. From my point of view there shouldn’t be a problem to start flowering when they are different ages unless they both had enough time in veg. Maybe the younger one will bring a little less yield…
And for your Co2 things I would recommend you to read this thread: Natural Co2 generator!
Grow looks nice to me so far.
Cheers