The 6 oils I usually use are Lemongrass, Cinnamon Bark, Frankinsence, Peppermint, Eucalyptus, and Tea Tree.
(I'll sometimes add oregano, Mrytle, or Lavendar)
I agree that Young Living is ridiculously expensive. That's who I originally used. After lots of research I have found that Aura Cacia is a decent brand but more of like a "maintenance" quality. My favorite brand to order from for price and quality, and they are all I use now, is Aeromatics International. Another brand I had as backup was Floracopeia but they are more expensive than Aero, but less than YL. Aeromatics ended up being about 1/3 the cost of Young Living for the same oils. So I've been using them for 8 months now.
I did use oils while I was on cayston and tobi and did not notice any particular reaction. I know this sounds crazy but I haven't been on inhaled antibiotics, nor IV nor oral antibiotics, except for Azithromycin, for about 6 months. Simply because I gained 35 lbs and lung function went up 18% when I started using essential oils so i said I would rather save the antibiotics for when I really need them rather than building up resistance to them by using them all the time. And luckily I haven't been sick yet except for right after I had sinus surgery a few months ago. I DO NOT RECOMMEND stopping your inhaled antibiotics, I'm just sharing with you my experience and reasoning.