They do eat algae. And if it is there, they will graze all day off the rocks. They don't seem to like diatoms and other thin films in my experience though. And even a thin green sheen would ruin aesthetic of a fully black and white tank. That's why I quickly discounted it.
So, I come to aquariums originally from a reptile and amphibian background. I mean, it's been a long time, as I worked part-time in an aquarium store in the late 90's in my youth. But I still tend to hold the herpers ethic, and don't really "co-hab" species. I know co-habing species, or community tanks, is the norm for aquarists, and I pass precisely zero judgement one way or the other. But, I say this to make it clear, my experience with compatibility for co-habing species of fish is all second hand.
My experience is that both of those are pretty low-key species/types, so that should help. The saulosi are a bit smaller though, if memory serves. Were the saulosi the more aggressive of the two (and it might just be down to individuals between them to be honest), then I would think if you got the really nice black nagaras it might help reduce aggression. But if it turns out the larger acei are more aggro, then it may not matter as they may still recogonize the patterning on the saulosi males as a threat.
If I wanted to do any mixing of species with different sizes, I would likely try to mitigate potential conflict with rockwork design. Have rockwork that has spaces too small for the anticipated largest males to enter. It offers a respite from their shenanigans for the smaller fish. Honestly, I do this anyhow because I like to increase fry survivorship (in that case it's like large gravel hides for them).
