Blending Pathfinder and Iron Heroes classes will be difficult, the rule sets and design principles are very different. Having said that, I can give you some advise from my experince. Magic using classes from IH will be horribly outclassed by the PF classes. So, if you want to see the IH classes, you may have to prohibit the PF classes. The other classes from IH can compete but they need to prevented from having magical items or they will overly competitive. Example, a 20 level weapon master rolling out of bed and grabbing her favorite weapon can have the equivalent of a 30 AC. Her fighter counterpart can not begin to compete with that. The other problem is difference in PF feats versus IH feats. Just one example, Weapon Finesse 2 allows the weilder to use their dex bonus for both to hit and damage. Armor will also cause you some problems and here I recommend using one system or the other.
As a heads up a L20 melee character can pass 30 AC relatively easily.
10 base + 9 from armor + 4 from shield is already 23 before any armor enchantments (up to +5), a ring of deflection (up to +5), an amulet of natural armor (up to +5), any other defensive equipment or feats.
That's if you are building it as a armored character rather then a Dex character (which, particularly because of high dex, dodge, and mobility can push 40 against attacks of opportunity).
However, I agree overall with what you said. Pick one or the other.
It does sound interesting to attempt the mix and match. I always enjoy variety. Maybe if you count IH character bonuses as enchantments, it would do a lot of in the way of alleviating the worry about magical items.
The owner of Iron Heroes was woking on PFing Iron Heroes but I have not folowed their progress for almost a year. You might consider going to the website and seeing if any of the ideas for version 2 are usable for your conversion.