I think these rules should be reviewed again.....as they are good ones.
It definitely can take them longer to acclimate and accept food but most of the time, given that they come from a reliable source and in the right environment, they should be eating sooner than later. 2 small discus is going to be overwhelmed in a 90 g tank and adding on the additional stress from everything else; off course it'll take them longer to eat...Can the 2 fishes rebound in a few more days? Sure, that's possible, but why are we not providing the best possible environment for them from the start? Why wait and patch things up as we go along....
The person is obviously a beginner and we want to steer him in the right direction. The thing that worry me is the source of the discus as well as the attempt to keep juveniles in a planted tank, not to mention the other tankmates. Maybe it's just my eyes but that blue diamond looks a bit stunted, pelvic fin could be a bit longer than it should be for a 2.7 inches discus and the eyes look a bit big as well. Growing a discus out in a planted tank is more challenging and I just see a lot of potential issues for a new person. It'd be best if he/she can take it back sooner rather than later and really decide on if he/she is up for the task or not.
It's not the optimal living condition for both the discus (tankmates, only 2 fishes with addition of 1 each week, we know how discus can be if you add a new one each time and they might start picking on the new one and add on undue stress which can be avoided if you added multiple or all of them at once..., and given all the hiding space, it might be so stressed from the bullying and just hide all day and starve itself to death) and the other tankmates (looks like white skirt, guppies, and danios all subjected to 29 C temperature). The increase in metabolism will end up having the guppies die sooner and make the danios even more active than they already are (tackling the food faster and swimming around faster), which will stress the discus out even more.
We can take it slow and take our time with the research before the fact, but it's already after the fact.
I see your point on throwing out too much information all at once but let's be realistic and illustrate the whole picture of what is to be expected in regards to caring for these fishes. They're not hard to keep but they're not easy either. They're not that cheap and I don't want to see anyone setting themselves up for failure by not having all the information needed at their disposal. Be it that not all the information is directly relevant to the OP question, it is still relevant in the grand scheme of things and the overall big picture.
I don't mean to sound rude but if we have the knowledge and ability to foresee possible issues, we should warn newcomers about it. Then it is up to them to take it or leave it.
Think with logic and rationality more than emotion. Act with moderation and consideration. Contemplate ideals and realistic goals and weigh out possibilities and options. Temper not with personal delusions or false hope but learn to accept and move on.