On Earth 4 I just made a switch from an alliance who's main chassis was infantry so I currently have 12 squads of range and armor infantry units!
.... but the alliance I moved too, main chassis is vehicles! So what will happen if I attacks with my new alliance against Relics and enemy colonies!? Will my units be targeted first because they are the minority?! What will happen?!

If you are inf and they are vech

You both will be targeted at the same time BECAUSE, relics feature all 3 chasis gaurding them

The tanks will shoot at you

While the relics inf will shoot at them

Normally it depends what chasis the enemy has, but since your fighting a relic you both will be shot at

~ IdrA

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!?

Correct

yep the units will focus on the ones that they deal the most damage to. With that said, concussive (100% damage to inf) and explosive (50% to inf and 25% to veh) units will focus on ur inf, while beam (25% to inf) will focus on ur team's veh.

plus, ur armor inf won't protect ur team's veh, and ur team's armor veh won't protect ur inf. So u'll find ur range inf dying while the team's range veh is still alive and good.