It sounds like you're not breaking cardinal rules so much as adjusting priorities, - which is normal and necessary.
Cardinal Rule
#1 , - there are no cardinal rules.
It is clear from this statement that you have an addiction and are in denial. Own it.
Try explaining to a non-enthusiast how the word "only" is appropriate when describing the number of bikes you have.
I'm not trying to talk you out of owning that many bikes, it's just that the rule about riding them regularly is something I perhaps misunderstood. If the rule is really:
"If I don't ride it regularly, it's time to get rid of it" then I think it's a good rule. If the rule is "Crap, I haven't ridden the MTB in 3 months, I need to take it out this weekend." then I think you should scrap that rule.
If training for this event in January means that you're spending more time on your road bike (or whatever) at the expense of the others, so be it. If these events become a regular thing that you enjoy and you find that you never ride some of those other bikes, again, so be it.