I am thinking the Operative will be best suited to the Light body type, being able to be fast and agile will help to infiltrate enemy positions.
My Medic will probably be Light as well, being able to quickly get to your team mates and heal them while avoiding enemy fire.
I believe the Soldier will be best suited to the heavy type; one: because heavy weapons seem best suited to a solider, two: being able to withstand enemy fire while placing explosives, assuming damage doesn't interrupt the process.
And Engineer: Medium, one: because Medium is the odd one out, and because the engineer doesn't as far as I can tell, need to be particularly strong or agile either way.
What are your thoughts and plans on this matter?