I use a hot fill process for all of the sodas, I make a sugar/water solution and fill the kegs, then chill/carbonate. Some flavorings go in during the boil if they need that for extraction, some I wait until after it is chilled and carbed to preserve the delicate volatiles.
As we expand I might look at switching to carbing in our carbonation tank (that I only use as a HLT now) and cold filling. For now though I am not making enough per batch to use the 4 bbl tank, I typically only make about 20 gallons at a go.
I carbonate at 24 psi and 40F, for about 3.5 volumes of CO2. It would be nice to have a little more flexibility, but I'm going with something that should be good in a variety of sodas.
Sorry for the delay, I was swamped with stuff for the kids birthday and now I'm sick.