So we accept here at Leaf Cutting HQ, that very large colonies will pretty much take anything that is offered to them. I cannot remember the last time we tried the biggest of our colonies on a new type of leaf and they refused them. However the same leaves offered to the small colonies will often be completely rejected. And its not a toughness thing either. Sometimes the leaves are clearly of a texture that should be attractive to all of the Leaf Cutters but the small ones dont take them.
I am sure this is down to appetite. Leaves in the wild are not a limited resource in general. Access to suitable leaves is fairly well guaranteed. But in captivity we feed the bigger colonies enough leaves to do them for a few hours and then, normally, they run out. So the big colonies are always hungry, looking for more. If we were to place an established favourite, like Privet, alongside one of these new leaves, I am sure the big colonies would not take the unpalatable leaves until the favoured ones were gone. So its a matter of putting up with the unpleasant taste because the fungus the big colonies have needs the food and the ants have to deliver. Small colonies can ironically afford to be picky because their fungus does not require the same volume of food so they can take small amounts or even not feed for a day or two until they come across something more suitable.
Good stuff that needs more formal investigation.