There is a case for taking things slowly (ish) - this is a marathon, not a sprint. It can be beneficial to introduce changes one at a time, taking baselines, so that you know whether what you are trying is working.
if you have just started ABA, and then add on retained reflexes and dietary changes, and see big improvements - how will you know which therapy has brought about the improvements?
that said, you could always read up on the dietary stuff - these tend to be bigger, physical changes (such as 'better', more regular bowel movements) which are less likely to be brought about by other therapies.
I started with biomedical interventions just after 2, and saw great improvements. but then all the signs were there that diet was an issue (heavy night sweating, black circles under eyes - always looked overtired even after sleeping 12 hours at night! - low pain response, only ate gluten/casein containing foods, bright red cheeks/ears after eating gluten/casein products, etc)