My Congos were fine with my polys, and
clm08k
has them now. I was able to get larger ones to start, which helped, and none of my polys were fish killers. You could also try white tipped tetras (Hyphessobrycon bentosi) which I had for a while before my single Cteno started chasing them - he was a bit of a jerk to them fore some reason as a solo Cteno. I think with you having a few of them together it'd be less likely for them to care about other fish that were too large to be eaten. I don't think mine was the 'norm' in that regard.
I'd stick with 3-4 ctenos in a 90 gallon, personally. And I like the schooler idea as they are often the only ones adding regular activity to the tank.
If you want both Sen and Albino Sen, I'd do one of each of those, a Del (the looker) and a Polli (active and personable like the Sens).
If you wanted more of a different looking poly in there that might be less active / more of a hider but cool, you could do Moke or Palmas Palmas. My own opinion is that the Buettikoferis don't look all that different than a Polli and they tend to hide, while Pollis are more in your face. If you go for a hider, go for a pretty one.
Just my 2 cents.