I originally thought they couldn’t be because the first Zootopia film only has sentient mammals, not other sentient animals. However, the second Zootopia movie reveals they do have sentient reptiles. However, Sing also has sentient spiders in a montage of auditions. In Zootopia 2, worms clearly aren’t considered sentient because Judy and Nick were willing to eat them (they were grossed out but they didn’t act like it was murder) so it seems unlikely that spiders are considered sentient. But maybe I’ll be proven wrong again in another sequel.

So I think it is very plausible that Zootopia and Sing are set in the same universe. Culture and relationships between different species may vary from one city to another. Zootopia may be prejudiced against reptiles but the city Sing takes place in be more accepting.