Yes and no. Using packages from different sources can (and does, I’ve tried it) cause conflicting dependencies leading to failed builds. For example: package A from Core and package B from nixpkgs. A and B both depend on C. Package D depends on A and B. The build for package D will fail because there is a disagreement on what version of package C to use. So using it as a source of extra packages is fine, but as a dependency on an aux package may cause issues.