I'm coming to the end of my fourth week on the regimen and I want to add AHA in to my routine at some point. I still have acne and am still breaking out - though my breakouts are less frequent and are usually small whiteheads now but I've still got a few papule type spots on my chin and jaw.
Should I wait until my skin is clear before adding in AHA? And does AHA replace moisturiser?
Also, are there any store bought alternatives to Dan's AHA? The closest thing I have to it is a product that has AHA and BHA and is marketed toward fading acne scars. I haven't used it yet and bought it before I realised it had BHA in it... I'm probably going to try it at some point but if anyone knows of better alternatives to AHA that I can find in stores, it would be great if you could let me know 
I think you should wait a week or two before you start using it. And when you first use AHA, you have to use a pea size and then work your way up just like you did with BP. Last week, I tried one pump on one side of my face and another pump on the other side and I looked like I got sun burnt. :/ Not cool so I only apply one pump now every 3 days although I've been using the AHA for almost 4 weeks now. Just don't use too much at first.
No you don't have to be clear before you start using AHA, but you do have to make sure that your skin has (or almost has) adjusted to BP. If you start too soon with AHA (or too fast with every day use) it will just leave you with a red irritated face.
I can't answer any of your other questions, ive only ever used Acne.org AHA.
I would say you're well enough adjusted to Bp to use aha when you no longer get intense redness to areas of your face and you no longer get an intense burning or itchy feeling after moisturizing. Dryness and flakiness is fine, plus a slight redness to your face overall is fine and its usually ok to start using aha.