The 2C-B may be slightly safer only because it is a *little* less stimulating IMO, but the difference may not be that big. Also it depends on how you equate potency / dosage scales of these two compounds. I've never heard of 2C-I being a MAOI in particular, but maybe it is a substrate for MAO-B? Not sure.. If it is I imagine other 2C's are as well. Sources for these claims / this info would be helpful - or are you going on rumor?
DMT doesn't work very long and basically has very little lasting effect on physical health - it has a reputation of being pretty safe like many classical psychedelics -, but it does temporarily work on your heart and blood vessels pretty hard, you can peak pretty acutely.
On it's own for a healthy individual that should be fine, also my opinion and limited experience with taking DMT on top of a very moderate reasonable dose of something like MDMA is also fine.
However when you start talking about a 'fair quantity of mephedrone', that to me sounds like you are going to party with it, redose and/or dose high etc. Even if you pick 2C-B instead of 2C-I, combining it with mephedrone can be expected to cause synergistic effects, augmenting the consequences for your cardiovascular system... stacking DMT on top of that as well seems like asking for a decent chance of getting real heart problems to be quite honest.
So afaik 2C-X doesn't immediately cause dangerous interactions when combined with stimulants or euphoriants, in fact I have taken 2C-I with methylone and K before and it was fine - pretty strong experience though..
The ketamine isn't really a hazardous issue in this potential mix, but it doesn't help if you become dissociated from your bodily feelings which could make you less aware of dangerous signs your body may be trying to convey to you. Also euphoriant drugs like mephedrone are also known to have a numbing feeling, if only because the euphoria overrules distressing feelings.
So all in all, there is no direct contraindication that I would know of... but if you can't keep yourself from redosing mephedrone - and most people will have a tough time exerting such self control! - you'd do well to really limit your intake of other drugs that add to strain on heart and bloodvessels. Be realistic about how much mephedrone you will be taking, and anticipate a scenario with excesses... with 2C-I you have to plan ahead, but with the DMT you can decide pretty ad hoc whether it should still be safe enough or not.
Be careful.
