^ Actually, that's why I think the Lizard could work.
If you had members of the S6 that we knew, like Rhino or Lizard, as you say it's easier to fit them since you don't have to go into their back stories.
But if you have some of the S6 be ones that would turn on the others, it makes it even easier.
For example, if Black Cat was one of the S6, since she is hired to steal for them, and she develops a "relationship" with Spider, and then at the right time she turns on the S6 guy who has been hitting on her and/or treating her disrespectfully. Someone like Rhino. I could easily see Rhino smacking her bum, or something, and then at the right time she takes him out, says some good one liner, and then leaves. This takes care of 2 of the S6 and it's not Spidey having to beat every one.
Same if Doc Connors is swayed to be part of the S6. So many ways to do this. Threats, promise of freedom, promise of power, promise of science, etc. He comes to his senses and either helps Spiderman, maybe takes out one of the S6, or just slinks away.
Once again, good ways to get rid of multiple members of the S6.
To me, the main issue with using Electro is that he was just defeated. He is obviously going to be a huge threat in TASM2, so bringing him back to be a lesser threat is questionable. Conversely, Harry-Goblin isn't a main threat in TASM2, no matter what he causes to happen. If he gets away, as I expect he will, bringing him back to be part of the S6 in the next movie is simple and makes sense.
So, based on all of this I'm seeing the S6 as follows:
Doc Ock
Vulture
Harry-Goblin
Rhino
And two others, like Black Cat and Lizard.
If you think about it, by going with these 6 (even though most would want Mysterio, Kraven, etc.) you can pull it all off in one really good flick.