Is that really ALL the behaviourist said... what professional organisation were they with?
I feel like there is more to it than just that.. and I am a dog behaviour/trainer..
Feeding things that cause tummy upsets, gut pain, and cause hunger will likely have knock on behavioural effects, yes.
I don't think feeding a diet based on roast/boiled chicken and rice will do your dogs any favours though!
A dogs diet needs to be balanced - dogs are better adapted to (almost) a keto diet - protein, fat, very little in the way of simple carbs or sugars.
If you don't want to feed a raw diet, and you find the commercial completes that are high end in quality are too pricey, you CAN mix and match.
The idea that you must stick to one brand, one flavour, and never deviate or your dog will get a dodgy tummy... is a/bollocks and b/ a self fulfilling prophecy.. if anyone eats the exact same limited diet and then suddenly changes it, they'll get a dodgy tummy!
Take advantage of offers on high end completes, as long as they're mostly meat (but learn to read labels... watch out for foods that split the cereal content into three or four different types so that the meat content appears first as per the rule 'largest single ingredient first' - this means a food that is 75% cereal (24% rice, 24% maize, 24% barley, 3% oats) could be listed as '25% chicken'... and listed 'chicken... rice, maize, barley... oats.)..
Stock up on several small bags of decent food, raw, kibble, cooked wet food, suitable food on offer in the supermarket (chicken wings, heart, small quantities of offal, raw veg blended to the consistency of grass clippings (freeze it into icecube chunks).. and then feed something different every day.
If they don't want a meal, fine - make a note of what it was, store it or bin it, serve something else the next meal. If they refuse that particular food several times, consider that they genuinely don't like it (we all have things we don't like!) and avoid offering it again.
You should find that if you do this, you find what your dogs like, don't like, what they thrive on etc.
If you provide variety daily, with a good core of quality foods, then you will soon have dogs who are NOT fussy.. as they've no need to be!