I am very much doing this. I am going to use some human infantry (probably Empire) to represent by Myrmidian Legionaires. I shall also be using Stormcasts to represent the Praetorians.
I have a Scorpion bolt thrower on order - and that will use some suitable rules (perhaps Dwarf or Elf 'thrower). And you suggested a pack of wolves, using the Chaos hounds rules!
I also want an A-Bomb .... counting as an A-bomb
We found it and we have kept it in a cage and we throw it at the enemy, while cackling madly. It's not a subtle strategy . . . .
And I do want some kind of Steamtank - my army has a steampunk vibe, so I think that is very cool.
I think the "counts as" is a great idea - but I have something even MORE exciting!
It is "dukes as" (because Dukes outrank Counts and ... alright, I'll shut up).
What you do is take the unit in question (for example, Chaos Hounds) and then you reproduce the Warscroll changing the names of the attacks etc and giving it different (and more appropriate) keywords.
So, for example, for my army I would remove the "Chaos", "Chaos Warhouds" keywords. I would replace them with;
ORDER, MYRMIDIAN LEGION, ANIMAL
That is pretty obvious, but I could also take the Lord-Celestant rules to represent my Praetor. Now, she is a great warrior so perhaps the profile can remain unchanged - but the keywords would have to be totally changed (ORDER, MYRMIDIAN LEGION, HUMAN, HERO, PRAETOR).
This is a sort of half-way-house between inventing your own scrolls, and doing "counts as". You want to field the bear - cool! But you should change the keywords to ORDER, ANIMAL, OH S*** IT'S A BEAR.
You could also simply remove the ranged weapons - because there are no points, it is not as if you are gimping yourself.