Those rules apply very much to psychedelics. Also to dissociatives like DXM but I think it is a little different: on dissociatives you tend to feel removed from reality, from your surroundings and from what's going on. Also not the same things tend to bother me anymore. On psychedelics tripping in secret is a double no-no (thats 4 no's!
) because being so unfree has very amplified frustrating effects which can have all sorts of bad and anxious effects on the trip, some of which may get projected. On dissociatives this in my opinion is true too but in a more minor manner depending on the headspace you are in (there are varying dissociatives).
However on dissociatives there is instead another issue: you may be less aware of what you are physically doing including the noises you make and other things, or you may experience it very differently. This can make it hard or impossible to hide what's going on if things really get that bad (if you really make that much noise or if all noises are really that audible to others - it may not even be from puking but from just speaking or making random sounds because you aren't quite aware of what's going on - thing talking in your sleep).
10% of white people naturally have an enzyme deficiency that makes DXM significantly more potent to them. It is therefore recommended against taking more than 300 mg the first time just in case you are one of these people. It's not a racist thing, lol, but a genetic one... other races may have higher or lower prevalence of this deficiency.
You should really find a better opportunity if you don't want to risk getting caught and/or having messed up experiences. We've all been young and sometimes opportunities better than this one are nowhere in sight... so if you are taking the risk I'd personally keep it reasonable and start off with 300 mg. It's not (never actually) a pussy thing to do but a smart thing. Idiots get caught or get too fucked up because of overconfidence, but if you have a little patience and take it easy so that you can learn how to properly manage it, on the long term you can get the good kind of fucked up because you can use your experience to make proper decisions. That seems like a win-win to me, or even a double win-win. (Plus: you lower your chances of puking - with 300 mg you actually have an OK chance not to puke hard)