If you are planning on running real slalom gates, as opposed to Nastarish slalom you need a slalom ski. Period. Even trying to run a cheater (worst of all worlds IMHO) it will still be simply an obstacle course. Not just slower but absolutely no fun.
And as was mentioned above, running a slalom ski in a GS course can not only be scary but present a very real risk of danger. If it hooks up on you at GS speeds, things happen very quickly and you can get launched (ask my rotator cuff how I know....) .
Times have changed from 30 years ago when you could run one ski for both. If all you are doing is playing about on simple club/recreational/Nastar courses you can get away with most things. But if you are using real courses get something more like the proper tool for the job.