The MMS 3rds have different chest, leg, and back connectors. Also, I think you'll crack a 3rd's neck piece if you try to jam a 1st head into it... or was it the other way around...? Anyway, MMS 3rds, both Tall and Small, are not compatible with EX Sets. Those absolutely require MMS 1st bodies.
And don't worry too much about the mods; they're not terribly picky when it comes to answering questions like this. Just to be safe, post more pics of your growing Shinki army!
