You have to have at least 15 hours in category/class for ANY plane you want to instruct in. If you're giving multi instruction for a rating or operating privilege, you must have at least 5 hours in make/model.

FAR 61.183 says that you need 15 hours PIC in category/class before you get the instructor rating for that category/class. So you would need 15 ME PIC to get an MEI. Also, 61.195 says for ME, you have to have 5 hours PIC in make and model to give training for the issuance of a certificate in a multi-engine airplane.

As I read it, you would have to have the 5 make and model, but not the 15 category and class. You need the 15 hours to get a MEI, you need the 5 hours to give training for multi engine instrument rating.

Feel free to correct me if I am wrong.

Ok, so if I am giving instrument instruction in a multi-engine aircraft, and I have 15 hours PIC in airplane multi-engine land...

Would I need 5 hours in the make and model if the training is not for a new certificate or rating? Say its just for a IPC, or maybe the pilot flying just wanted a CFII to keep an eye on them to make sure they are flying safely.

No. If you're just doing an IPC or acting as safety pilot, you may feel free to hop into any multi you desire (as long as it doesn't require a type rating).

To answer the 15 or 5 hour question, the way the DPE explained it to me was that you 15 hours of ME PIC for the rating. So if you are getting you MEI in a seminole, you need to have 15 hours PIC in the seminole. Now if you have your MEI and someone calls you to teach them how to fly a Baron, you need at least five hours in a Baron to teach the student. If someone calls you to teach in a Seminole you have the five hours PIC already so you can teach them. I hope that makes sense.

Close, but not exact. The 15 hours PIC to get a MEI don't have to be make/model. You can have 10 from a Senica that you got your MEL in and then 5 from a Seminole that you did your MEI training in. It doesn't have to be the same for that requirement. That is only category/class. You are correct about the 5 make/model to teach out of the plane, though.

It is all so clear to me now, thanks everybody!