Hah! Love the idea.
First up, I think the ramifications of a rat as smart as Tony Stark are going to be considerably larger than the ramifications of the Ironrat Mk1 suit.
Anyway, that aside: lets look at the differences.
1: Small things are stronger for their weight than big things.
In purely structural terms this remains true. The suit will be able to transfer a greater mechanical load for its weight than a comparable human sized suit. Where this falls down is the motive forces for the suit. Depending on the technology used (Electric actuators? Hydraulics? Pneumatics?) the control packages required might weigh more than they allow the suit to lift. While a hydraulic 'muscle' can still output more force/kg than a larger version, the smaller muscle will still have to have an equally powerful pump in order to maintain that ratio.
2: Small things are more impact resistant than large things. This isn't quite true. Smaller things carry considerably less momentum for the same speed of impact (a spider falling off a roof will be fine, an elephant not so much) and if a small rat is hit by a small hammer it will do less damage than a big human being hit by a big hammer moving at the same speed, however impact resistance depends very much on the concept of impact. If Al falls off a roof, then as he is light, has a low terminal velocity and carries less momentum then he will be OK when his velocity inevitably changes upon impact with the ground. However, if Al's weight has been doubled by the suit, the velocity change at the bottom may be enough to push Al's internal organs through each other while leaving the suit intact. There's less space in a tiny suit for shock absorbers, and the relative mass gain of the small suit may well be greater than for the large suit (again realising that you can't just scale components down). The point I'm trying to make here is that the Ironrat Mk1 will be better at stopping tiny rat-bullets than it's bigger brother, but worse at preventing massive haemorrhaging after a sudden stop.
3: Small things are faster than large things Now we get into the difference between speed of thought and of movement. Turns out that in the case of our suit the two can be decoupled, depending on sophistication. If your exoskeleton is partially automated then an act like running on water can easily be performed. Just let the suit know and it'll slap Al's tiny feet into the surface tension like a jackhammer with claws. While the human equivalent of this suit would pulverise the knees of whoever was inside, Al's knees might well survive as his suit would be lighter, stronger for its weight and even able to transmit the necessary control signals faster (though that last point is negligible), which would reduce the amount of pounding necessary from 'pulverise' to 'mildly uncomfortable'. However, if your suit only moves as fast as Al does, then he's unlikely to be able to do anything he can't as a rat. Even though the suit would make water seem like he was running on treacle water is still a Newtonian fluid (unlike Oobleck, which he would weirdly find harder to run on than his human counterpart) and his mass with the suit is more than sufficient to break the surface tension. He might be able to swim fast enough to start hydroplaning if you designed the suit right though, which would be pretty epic!
Uses: I wouldn't like to fight Ironrat. Just imagine a metal rat with neurotoxin tipped claws trying to climb up your trouser leg. Shudder.
A rat operative with the suit on would be much more overpowered compared to normal rats than a human in a suit would be compared to other humans, capable of shredding through cage doors and quite literally crushing opponents between their paws. However the suit wouldn't help against even a normal human. A rat that's 20 times stronger than a normal rat and only weighs twice as much can still be picked up by the tail (Never, EVER try to do this with a rat that isn't wearing power armour), not to mention the risk of kicks from steel-toecapped jackboots. Oh, and while IronRat Mk1 is good at stopping tiny rat bullets getting hit by even a small amount of shot from a shotgun is still going to be like getting hit in the chest by a football made of lead and moving a few hundred miles an hour.
TL:DR Ironrat Mk1 works better than expected vs other rats but is still terrible vs humans, and everybody should be scared of Al Stark's weapons empire.