..I thought the basic was the CFI then the MEI was an add on
Not true. You can do ANY instructor rating as an initial. Glider, helicopter, MEI, etc... doesn't matter. After that any additional instructor ratings are just that... additional. You just have to hold commercial privileges in the category and class of a/c you choose to do the CFI in. Also you have to hold instrument privileges in that category and class if you want it to be an instrument instructor checkride.
Most of the students I worked with as an instructor did their CFII first and then added on the CFI.