Nope...will not. Perhaps if by big ass tank you mean over 1000 gallons. If that was the case you would be able to school rhoms to. In saying it will not work I assume you do not mean 1000+ gallons. Even if you have a large tank by hobby standards (300-360 gallons) it will not work.
Well i'm not a Pro but a 14 years hobbyst, still, i believe i can help you...even in a small tank you can mix Pygos so if you really have a big one much better...try the mixed with Cariba and Reds and think about adding some Piraya if you can find/buy them!
I am no expert, but depending on the tank size, I think you could add a spilo cf or 2. It has been done before and I think could be done again. I just think you need to be aware that you may need to separate the spilo cf gets too aggressive.
this tank is gonna be 360. my uncle wor ks for a place that builds custom tanks and can get me reallly good deals on all the filters too, cleaning this thing is gonna suck but it will be worth it I guess. was thinking a ton of reds, but then wanted some caribe in the mix, but I think spilos are cool lookin so hell I don't know what I'm gonna do. this thing is so big it's going to take a long time to set up.
Maybe it will work out if you provide a lot of hiding places, get all fish at the same size, getting not nearly as many fish as your tank space allows to (for example: 4 reds, 4 cariba and 2 spilo's: 36 gallons per fish...), and taking into account everything that might lower agressive behaviour.
But again: you never know how it will work out, and even if it seems to work initially, it can turn into a complete disaster almost instantly.
I don't know if it's worth worrying sick 24/7 about your precious fish, even though it would kick azz if it indeed worked out
Do it! One spilo will not hurt the shoal. Especially if its a big shoal. It take experiments like this to prove people wrong who have never done it. So go for it. I know I would. But then again I like pygos to much to get a serra species.