CFI-ME certification
Yes, you have to take a practical test to obtain a CFI certificate.
In your case, you could take your initial certification in a twin. Your initial certification would be as a multiengine instructor. Your oral would include all the FOI material and everything else that is set forth in the PTS. Then, for the MEI, the examiner would test you on multiengine procedures and principles, e.g. Vmc, aircraft performance, and systems for the test airplane. The flight would basically be a Commercial Multiengine ride out of the right seat. Do all that, pass (of course!), and, voila, you are a Certificated Flight Instructor with a Multiengine rating.
You can obtain initial certification as an instrument instructor in a multi. It isn't unheard-of for people to combine the MEI and II rides in one practical. In fact, I believe that some schools who train people for their Commercial-Instrument-Multi do that. They finish them in the left seat and immediately put them in the right seat for their CFI-I-MEI. I believe that ATA and/or ATP do that.
Hope that helps.