Low sprocket exceeds derailleur max teeth but total capacity within range of derailleur
If the RD is suited for 36 teeth it may work with a 38 but highly unlikely with a 42. Shimano is conservative with teeth counts, you're right but still there are limits to get things to work. It is mainly a problem with geometry. The cage will have trouble to take up the extra length of chain. And also not to get into the way of the sprockets. You could try with one of these devices that put the RD 2cm lower but you'll trade it for less precise or more sluggish shifting.
The size of the front rings plays a lesser part if the difference the two is between more or less standard, although with smaller sized rings you may have trouble for FD height with a braze-on mount.