Keep a close eye on the pH of the Promix as it get's older- they mix lime w/ the peat to buffer the pH up- peat is naturally very acidic. Eventually the lime is consumed and pH starts to nose-dive. No big thing if ya watch for it.

The compost must act as buffer in the Calloways ( or maybe the shale- very broad term)- otherwise you'd be treating the water the other way. Prob a fair bit of compost, or at least a fair bit of surface area.

Go ahead and use the Calloway's- just cut it with a bunch of perlite. You'll have consistant chemistry, makes it much easier to manage. Multiple media is a headache that you can avoid. Perlite's inert, so chem remains the same, just dilutes it. will drain much quicker, which is usually a good thing.

Hmm- album title in that... " The Chem Remains The Same".