You can get the CFII first or you can get the MEI first. Don't look at CFII and MEI as adds on to the CFI. You can get any instructor rating first and the others will be adds on.
Some people choose the CFII first because there aren't as many lesson plans to create. Either way it's personal preference. One thing for sure is there is more work for CFIs than CFIIs. If you get the CFII first, you definitely want to add on the CFI. Most students you get will want a Private Pilots Certificate.