Why would Riza be fighting Roy? The Ishbal war was against the people of Ishbal. Besides, if she was trying to shoot Roy, she would have done it. It was a clear shot.

As for Hughes, I don't think they're the same. Hughes' knives have handles, don't they?

They are similar but not identical.(they're still push knives) Their styles of using the push-knives are, however identical (save for Mei Chan uses them for alchemy sometimes as well as combat) and it's curious since in the FMA world, methods, objects from foreign lands are rare in Amestris. Almost everything in Ametris is made in Amestris so that's why it's a bit odd that Mei Chan is using push knives like Hughes with similar skill as she's from Xing and we've seen that she's never been to Amestris (so far) until now.

Plus Hughes, throughout the time he appeared in the manga post-war, never carried a gun. He always had those knives. No alchemy, no modern weaponry. (Actually did he have a gun in the flashbacks we see of the Ishbal war?) That's pretty strange since EVERY member of the military (excluding secretaries and suchnot) has a gun on them, including Lieutenant Colonels. Perhaps Hughes had to learn how to use a gun to enter the army. We don't know the military's recruitment policies at all so for all we know it may have to be demonstration of ability to use a gun or demonstration of ability to use a weapon other than a gun. (Perhaps he wanted to be original and demonstrate his knive-throwing?)

That's my 5$ anyways. It might not have a relevance in the storyline...or maybe it will i hope so.... XD;

As for Riza, that wasn't a plausible theory I put down anyways. I was just going over various scenarios in my mind. xD;;