Personally, I think it'd be preferable if there was no "frankenspriting" or editing involved. That just seems like, well...taking the easy way out in a non-professional way. 
I agree. I want some totally original sprites.
But at least this is the start of things. You tried, and thats what matters. Keep the ideas coming everyone.
Crossed fingers, but, here is how I see some of the bosses. If you like any of the designs, please make a simple vote in your post. I may have added a few extra colors that aren't in the game, but you get the idea. I will make sheets to any bosses that make the cut. 2 of the bosses are not created yet.
Some nice ideas there, but they're too animalistic. Look for example at Pluto. He was still human looking, everyone should still have humanoid faces, even Uranus was human looking, he just looked bulky and happened to have bulls horns.
On the plus side, I like the colours you used for Duo in the corner. Perhaps with some changes to his head that may become an acceptable sprite to use for his Gameboy appearance.
I was bored and made this. IF you want it, possible Rigel design?
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I recommend we start with designs and sketches like with Invader_Rocks picture. We need to start small with hypothetical sketches and work our way up from there. Thats a good start for Rigel. But perhaps she's looking a tad too generic and reploid-esque (kinda like what people are describing Quake Woman as being, in the Archie comic previews)
One small trait and detail I imagine she'd share with her brother, Terra, is that mass of hair he had
EDIT: However on further consideration, she does have an intruiging look about her. But maybe she looks a little too friendly. I ponder if she should be the type of Stardroid who has a reputation for being beautiful but deadly. Using her looks to get what she desires.
PS. I've added Rush, Mega Man's trusty canine companion to the first post!