WhiteJoker
Theoretically Zero and X could beat Inuyasha. They fight similar opponents on a regular basis, and their weaponry is quite powerful. Of course, the question is which version of X and which version of Zero. X has his multitudes of X armor, as well as countless powers inherited from his defeated opponents. Zero has his regular form, the Maverick Virus within him, which, in essence, is a berserker fury mode, and the version we see in Megaman Zero, with cyber elves and quite a diverse assortment of weaponry. Theoretically they can win, but the question is, how powerful is the Tetsuiga? And do reploids count enough as human to stay the transformation of the Tetsuiga? Or are they like rock, which means the full might of the Tetsuiga (minus the scar... unless Maverick Zero's aura counts as demonic...) is put into play, and while the Tetsuiga is no plasma blade, it is still a mighty weapon most probably capable of blocking even Zero's Z Saber.
Let me look at X, his X1-3 armor would be out because there isn't much to it, you could add the Houdoken (however you spell it) from X1 or the Dragon's Fist from X2, but I don't think it would help much. My best guess would be his X5 armor, it introdues the Gaia Armor which is impervious to spikes and other one-hit death items. It is also very powerful in strength and defense, however it is very slow. The Falcon Armor also comes into play where X can fly for a short amount of time, enough to get away from Inuyasha. I would not suggest letting X use the Z-Saber (or is that in X6, can't remember) because he can't really use it, it would slow him down. If you give X the Falcon Armor from X5 and the weapons from the various Irregulars it should be enough the take him out.
As for Zero, I don't know much about Megaman Zero because I really haven't played them. So I can't speak on behalf of the Megaman Zero form, unless someone can add something. I would suggest Zero in his X5 form, this is when they first introduce him becoming an Irregular due to the virus. Also, you can equip Zero with the black armor which increases his defense (and attack a bit I believe). Inuyasha wouldn't be able to sue the Wind Scar because the aura isn't a demonic aura. As for the Z-Saber, I believe it could fend off the Tetsuiga, Zero is a master swordsman so that's a plus. I don't think, when pitted against the Z-Saber, Inuyasha could do anything with the Tetsuiga.
As for the Tetsuiga being used against the two reploids. They are robots with human attributes, unlike classic Rock who didn't express (to a point) human emotions; these two do. They were designed that way. I don't think the Tetsuiga would be able to work against they two because they do portray human emotions, unlike the classic robots back in the classic Megaman series. The good thing also is that Inuyasha, while half human, is also half demon. Therefore X and Zero wouldn't be breaking the first rule of robotics, which would be that no harm can come to a human by a robot.
I think the two would be able to beat Inuyasha. They have the speed advantage in dashing, air dashing, and teleporting. The teleporting would also throw Inuyasha off his game. They have the advantage in range and power, seeing as how the Tetsuiga really can't be used he would have to use his claws, X and Zero could gain their distance rather quickly and fight him from a distance. Power comes from the ability to charge up not only their busters but the powers they retrieve from the other Irregulars. I also don't think Inuyasha has experienced plasma type attacks, which could probably cause more than normal damage to him.