I see what you did there
Frerin is right, you would just need to swap troops for each warband when not playing friendlies, but that is not the purpose of this topic.
Yes, the Grimhammers profile is a little disappointing, hopefully they will be reviewed shortly after the last film. They are actually worth the same points as Khazad Guard as they have -2 stats (2pts) but have throwing weapons (+2 pts) so they are actually on par. Throwing weapons are quite underrated, so I wouldn't discount them but do some playtesting first.
The Iron Guard i think look good already, they are only "Iron" guard not mithril or steel guard though, so are actually not that highly ranked "lore" wise. If you wanted to add something to them, my suggestion would be to use green stuff/putty to sculpt some leg plates over the cloth. Most of the other dwarves have these leg plates but they are basically built into the suit of armour. In the case of the Iron Warriors, they would need to be a loose attachment starting around the waist and covering to the top of the knee, but no further down. I would suggest using a chevron or ^ style armour for this case.
Look at the grim hammers/GH captain. You will see around their hips what I am talking about. Just move that style armour to be covering the top of the leg more, not the hip. This could be "strapped" on at the top of the knee and through the waist belt if you are concerned to that degree of things.
I don't know what you mean about having helms, the Iron guard already have them. I would leave them as is. To give them a more "iron" feel, you could just not have any gold on the armour, but differentiate the metals by darkening some and only making it like bright steel on the edges of things like swords and maybe a faint line of it on the chainmail.
You can't do much with the warriors - swap an axe here or there, the difference here is mostly how you paint them, so just give them a bit of a different colour and you won't notice the same sculpts being used.
If you were referring to warriors of erebor, I think the easiest thing to do would be to add the ^ armour again over the cloth on their legs and even on the shoulders of some you might think are a bit too bare.
Good luck!