^ It's difficult to find ANY same sex pairing that could be labled as "canon", considering that there aren't any.

And "slightly canon" I don't think really exists, other than just the female characters that have interacted with one another in some way or form. Haha. The rest is just a matter of having a good imagination!
When it comes to FMA manga yuri I think the most popular is
Riza/Winry. Not only is it sort of like the female parallel to Roy/Ed, but it is also interesting to note how they are shown to have similar personalities is the Gaiden "Simple People", where I think the pairing gets all of it's evidence. Other than
here, where the two call each other pretty in one little panel, and on the page prior, when Winry blushes upon seeing her.
If you're going anime1-wise, I think a very popular one, (and rightfully so) is
Winry/Sheska, especially considering that Winry and Sheska were both single at the end of the series, making their relationship past friendship arguable. They did a lot of things together in the series, but it's been a while since I've watched the old anime (I've been all about Brotherhood lately), so my knowledge of examples is a little hazy.
Riza/Rebecca I guess could work. They are, after all, very close friends, and I have heard of stupider reasons for a pairing.
Izumi and Olivier have been shown admiring each other ('s strength), but Izumi is just too dedicated to Sig for that to mean anything to me. (Shame too. They would make an awesome pair.)
In the same way that Riza/Winry is sort of like a parallel to Roy/Ed, I have seen Elricest fans who have stuck
Winry and Mei together. Though, the two don't really interact that much, so I would lable it as a "crack pairing". (Sorry. ^^)
Winry/Paninya is sort of like the manga equivalent of Winry/Sheska is the sense that "BFFs=lovers".
So there you have it! That's all the the "slightly canon" yuri pairings I can think of! I can give you PLENTY of crack-pairing that I think would be awesome, but let's not get into that... ^^ Hope I helped!