The issue with Al80's isn't when you're carrying one, it's when you're carrying multiple and the physical size/gas required ratio means you're carrying extra mass and physical volume for no gain. Stack 3 Al80's on your side and you can see how having two of them be Al40's would be beneficial. Obviously if you don't have access to more appropriate cylinders you make do with what you have.
For most deep bailout/deep deco, you're looking at higher volume required because of the depth. Once you get to your 50% and 100% bottle, you're shallow enough that you're not worrying about the gas volume, hence the preference for lower capacity, smaller physical size, cylinders, especially if you have to carry them the whole dive, or have to deco out in close quarters.
As for volume....
You don't plan 1/3rds or halves for your deco gas, you figure out how much volume you need to accomplish the dive and choose the appropriate cylinder based on that plus whatever you determine your reserves need to be. It's a conversation you need to have with your dive buddies in terms of lost gas planning. There are several schools of thought re-sharing deco gas, clearing yours then handing off, everybody stays on previous mix to keep the team aligned, etc. It may change based on the length of hang time too. You need to figure out what your plan is with your buddy based on the profile, the environment, etc. What could be as simple as sticking to your own O2 and your buddy on next richest until you're done with yours then hand off until he's done, to a three way split where each guy spends 10 minutes on, 10 off swapping O2 until everyone is clear, it all depends on several factors.
I know some rebreather guys that plan on a modified team bailout in situations where everyone carrying individual ideal bailout would be impractical, so everyone carries enough that if everyone has to bail, there's enough gas by volume, it just won't be optimal mix for every member of the team at all times. There's no one right answer.
Except 80% is a lame choice of deco gas. That one is certain.