That's exactly how I did it, and for the same reason. I did my MEI as my initial, and then went back and did my CFII and CFI-Single Engine as an add on. Check out the PTS task table for the specifics of what you have to do on each checkride and then for on the add on rides. Doing it this way doesn't necessarily limit information you're responsible for on the oral though....mine was still 8+ hours. And for the things you don't have to do, they're replaced by things you do have to do (or can be asked to do) in the multi (VMC demo, engine shutdowns, etc.) Personally, it was a good way to go for me. If you plan on teaching in a multi, definitely do it. If you aren't planning on teaching in the multi, I might suggest just the single engine for the initial.