The way the battle system works out is whatever enemy units are setup with concussion upgrade (i.e. anti INF) will focus their attacks on your units before attacking those of your allies.
Its best to standardize unit types so my recommendation would be to simply start building the same chassis as your teammates but don't just get rid of your old units, they can still be helpful.
So it would be best if I left these infantry units out of mass attacks with my allies and just used them for attacking OPs and noobs with crystals!?