I was trying to understand why we needed a steel typing for this CAP. I asked in #cap and Korski said that he liked it for its lack of sandstorm damage, SR resist and immunity to toxic. Personally I was confused because I felt that the fighting and ground weaknesses from Steel were forcing us into mathchups such as Ghost and Flying and Poison to remove those weaknesses . . .
Well it's not bad to have a pokémon that has checks. But, someone (not sure who) said this earlier, but it bears repeating so I'll say it again: If you are a belly drummer, you behave, in some respects, the same way a Shedinja does: you stay hidden in the roster, and don't come out until your counters are gone, and then if the other team doesn't have a response left for you, you absolutely steamroll them.
So, In my opinion, the best 2 possible steel typings are
Steel/Fighting and
Steel/Fire.
Steel/fighting is weak to fire, burns, fighting and ground type attacks, but packs a VERY strong offensive typing as well as 2 STAB priority attacks, one with excellent coverage. And before the argument comes up again: yes it's important. It lets us direct our CAP into being a Belly Drummer without giving it enough attack to just be a choice band sweeper. But, again, lets look at what this typing means as far as steel weaknesses: it lacks any 4x weaknesses and can come in on a predicted bug, poison, dark or rock attack pretty much for free. It totally shrugs off stealth rock, and if the enemy pokémon doesn't have one of the three 2x effective attacks to smack you with, chances are, you get belly drum off (especially packing a Sitrus Berry) and then you basically steamroll with your Mach Punch, especially if you gain an ability that boosts it (with bullet punch in the wings for speedy ghost, psychic, flying, bug or poison pokémon), and with both of those attacks being priority, you become very hard to stop.
Fire/Fighting works rather similarly, trading a resistance to fire and burns and the ability to come in on grass, steel and ice attacks for free for a 4x weakness to ground, a weakness to water, no longer resisting rock, and, but that token, stealth rock. Also, you lack a fire STAB priority attack . . . which can be worked around, but is, nonetheless a negative point (though we can have the second best thing with flame charge)
But even it is a good enough typing, and neither typing is ghost or flying, as you seem to think that we need.
Yes, we'll have weaknesses, but all we need to do is make sure not to try to set up against one of those things we are weak against.