general rule in my group is it's allowed, we love apoc, but only play every few months and we've spent a bit of money on having legit formations so we allow them in 40k too. However, any apoc formation or super heavy you need to have your opponents permission while list building in our group, so that he can say no if he's not in the mood, or at least build his list to deal with it.

You can't really just say yes to all apoc formations, it's for sure a case by case basis that you need to discuss with your opponent beforehand. For example the demonette formation, even in apoc that formation is scary, in a 40k game it would be pretty miserable to fight, it is very cheap and powerful, it fits into small games and there's not a lot of good counter available at those point levels. While on the other hand, creed in his bastion is cool and totally fine in a game of regular 40k imo, as long as you don't allow the high command rules that go with it since those rules aren't in the brb.

I guess what I mean is nobody is forcing you to follow the rules when gaming with a group of friends, I think if you are wanting to blindside each other with the more powerful formations, I'm not really a fan of that idea, but if you discuss it before the game and both of you agree on using specific formations, they can make for some very fun and still fair games.