Hi and welcome to YT
! Bless your heart for taking in these dear ones. I'm sure they're in such good hands with you. They sound like a fun pair.
On the feeding...hmmm...it will be tough to have the chi's bowl out and not have the other 2 eat from it. The only thing I can think of is to have the chi in a separate area from the other 2 all day so the chi can have the bowl of food.
Not sure what you're asking about the 1/2 food and veggies. Are you asking if you can give 1/2 the amount of recommended food if you add veggies? If that's what you're asking...then no, I wouldn't cut the food in 1/2 as that not enough protein then and it would be unbalanced. I would cut less of the food.
On the pooping...I walk dogs at the shelter where I volunteer 2x a week, and these dogs often will just stop and poop whenever/wherever the urge suits them - and sometimes, that's right in the middle of a sidewalk or while crossing the street or whatnot. Dogs aren't picky...when they gotta go, they go.
On the oils - you can use things like lavender and other essentials. If you properly prepare them, you should be fine. I use "balms" that are excellent and they come in alllll these different formulations. We use these at the shelter too, you can buy them here:
BlackWing Farms
On the fighting thing, it's probably just a habit they have when they get overly excited and I wouldn't worry too much about it unless it seems they're taking it too far...?