With good support settings, orientation (rails up vs down) doesn't really matter as long as it's horizontal and not some weird angle. Rails up is less likely to have warping on the dust cover, rails down uses less support material (and is therefore a little faster).

But you really don't need to do multi-material to get clean support removal. Tune your support settings and your cooling and you can get clean support removal without wasting a bunch of time and material. Here's one I printed a few days ago, rails-up, in 3D Fuel Tough Pro PLA+.

I didn't do any clean-up other than just removing the supports. I could have probably gotten it even cleaner if I slowed the print down a bit, but this was only intended as a test mule.

You probably want a material with the least creep

I'd assume a PLA would work well with the PET-CF, but I don't know about the other two. Would be worth testing with a cheap PLA and/or PETG to see how that works.

petg is good for pla+ prints because it doesnt stick together at all. if you're using a bed slinger it can fling the filament right off of it though. the print towers and times are ridiculous if you dont have dual hotends. a 1 hour print becomes a 10 hour print when you use two different filaments like that. having to heatup the hotend to 250 then wait for it to cool back to 200 for the pla adds a crazy amount of time.