Whopity, correct - it was the reason for my original comment:
The rules are slightly different for a Class Rating Instructor certificate
A FI is issued without restriction on TMG/SEP class, whereas a CRI, for example, is issued on a licence as "CRI/For SEP". To amend that to include TMG will require compliance with FCL.905.CRI(b) and paying the CAA some cash to amend the restriction on your licence.
ifitaint...