Is it unprecedented to have some powers specifically not work with each other? I can't think of any off the top of my head, but due to the narrative of this power(Rock lee only having physical power, taijitsu I believe, in the show), it would make sense if the person couldn't have any other physical powers, and was limited in the powers they could take at all.
I don't think that there are any Powers that don't work with each other, except for Powers that are basically stronger/weaker versions of each other.
You could do it, but you'd need a good reason. And "this is a Rock Lee Power" isn't a very good reason, since people who aren't playing Rock Lee might to use this Power.
If it fills the toughness boxes, it's basically just gives armor, with some form of catch. Which is a waste of refresh, but since you aren't paying it's kinda moot. I think it works in terms of flavor, he becomes tougher as well as stronger and faster, but I don't feel the mechanics aren't quite lining up right. I do think it's getting closer to the right feel though.
I think it'd be better to just give armour. Less confusing.
Bear in mind, though, that the Toughness/armour will often be wasted because armour doesn't stack.
Some food for thought, with the current implementation, once they break 6 gates, they add 2 to their defense roll, and get 2 armor. For someone of the same skill for attacking as this characters athletics and no super natural bonus they will hit only 1 in 4 attacks, and less than 1 in 20 will they hit well enough to inflict stress pre-weapon. Since all they need is a stress of 1 to either force an extreme or take him out though, assuming at least a weapon 1, they will achieve that close to 1 in every 7 attacks.
At the extreme level gate 8, assuming attacks are coming in with weapon 2 and at the same skill rank, it's 1 in 20 to take out the power user, 1 in 140ish to deal damage without a weapon rating, and 1 in 7 to to hit with no stress, pre weapon.
At gate 2 and 4, it's a little less likely than a 1 in 2 and a little more likely than a 1 in 4, to hit with no stress, pre weapon respectively.
Looking at that I'm thinking gate 4 is a little light, and doesn't scale with the others, since a moderate sticks around for a session +, Maybe kick in inhuman toughness at that level with the current implementation?
I think that makes you too fragile.
When you've opened six gates, you're using 3 Refresh and pretty much every consequence you have. And in exchange, you get a 5/27 chance of getting splattered in one hit by anyone whose attack skill is equal to your Athletics.
You can probably safely assume that anyone worth going to six gates against has weapon 3 at least.
As for the eighth gate, it should basically make you invincible. You're killing yourself, after all.
PS: What do you do if you already have consequences when you want to use this? Can I take a moderate to use two gates? If I have a Severe, can I use eight? If I have a moderate and a severe, can I use six?