That's only half the story though. If it's a simple standalone package, yes it'll work. If it integrates with the system (ie. init, associations/mimetypes, icons, other package configs in /etc, dependencies) it may not work..
Alien should not be used to replace important system packages, like sysvinit, shared libraries, or other things that are essential for the functioning of your system. Many of these packages are set up differently by Debian and Red Hat, and packages from the different distributions cannot be used interchangably. In general, if you can't uninstall the package without breaking your system, don't try to replace it with an alien version.