That's not as challenging as you seem to think. For Nightfall, you could start with the assumption that there's at least one particularly massive star, not so big as a typical A or O that won't stay on the main sequence long enough for life to evolve, but bigger than our G 2 sun, say a G 4 or 5 or even something in the F series. The other five suns can be much lighter, all the way down to red dwarfs in some cases (and the story seems to describe at least one that is). Those small stars don't have nearly the light output of the bigger one - with the right options, The planet can orbit the main star at a distance quite a bit greater than Earth orbits our sun, and be close to the exact optimum of its "Goldylocks" zone or somewhere on the cool side. Then smaller stars could exist in various configurations, and their output is low enough that if they are at, say 5 x what that planet would call an AU, they would essentially just move the planet's climate a bit towards the inner edge of the "goldylocks" range. So long as they don't nudge it completely into the hot zone, why wouldn't life cope? (Note that we are talking about their light ouput raising the planet's temperature, not them gravitationally nudgeing the planet about - gravity and how stable the planet's orbit can be if the orbits of the suns themselves are changing, that's a seperate question) Fictional Kalgash would have to orbit the biggest sun of the group and it would have to count as being near the cooler edge of the life bearing zone before you figure in the other stars, but even before the lesser suns temporarily shift into a quasi-stable configuration that prevents night from occuring except once every several thousand years or whatever, there would be various configurations that would make night a very short lived or rare and irregular thing, and life would be used to that. There are other issues, such as how do plants dispose of waste products on Nightfall world, but those issues don't vary much if there's a short night every few months or only in a thousand years - plants would have to adapt for situations much less prolonged than the current one. If we call the Nightfall orbits "perfect", then even very imperfect multi-star systems would find life constantly facing this problem.I'm thinking that by your argument, it's all too easy to say things such as "Life in Binary systems? Impossible!," and even "Life when the day lasts more than 24 hours 17 minutes? Absurd!", and things like that. I'll refrain from quiting Jeff Goldblum at this point, but hope you will consider this.
Then there's the question of how sensitive to light the natives eyes are. If nights have always been at least short and irregular for much longer than the perfect situation has existed, we should expect the natives to not have very good night vision, as there's less demand to evolve it, so talking about relative optical wavelength outputs and such is very hard to do meaningfully.I'm not sure how we could criticise the work as SF on that basis.