I would drop the Oscar and go with the smaller Cichlid idea. The Oscar will just pollute the water and make everyone's life miserable -- they're great fish, but only in larger tanks. Also, it would eat your other fish -- well, the little tetra and shiner, at least.
As for the Plecos, be careful of which kind you get -- the common ones can grow huge and poop a whole lot, which will just make the tank nasty. You don't HAVE to have a pleco, either -- I mean, if you want one, go ahead, but some folks think it's like a prerequisite to fishkeeping that you have a pleco. Lots of us manage just fine without them.
Remember that you're going to want to up the number of Pictus and Tetras -- I'd do this first, anyway. I don't know what an Emerald Shiner is, but if it's some sort of minnow, it's likely a schooling fish, as well. And include some caves in your aquascaping for the Pictus.
How about six of your Pictus, a dozen of the tetras, six to eight shiners, and a pair of Angels? You can't go too large on which Cichlid you pick because they'll eat the little fish you've already got, and also, the Pictus, as I mentioned earlier, are going to represent a pretty large bioload if you keep them the way you should, and increase their numbers. This would also leave room for one of the Plecos that stay smaller, too, I think.