Is it possible to use old & new promotions at the same time? And if so, would it be reliable, or would you recommend against it?
No, that is not possible. Do you have old active promotions in your system, but want to develop new ones using the new system?
It's not supported out of the box, and I would advise against it. Having two promotion systems running simultaneously defeats the purposes of the new one, and it would be very hard to make it work reliable. Discount calculation is, simply hard.
The point of new promotion system is make it easier to implement the logic for discount, so I would suggest to convert the old promotions to new one instead. It'll take time, but it will be paid off in long term.
Okay, thank you both. You've given me perspective.. now I've got some thinking to do!
I'd like to begin setting up NEW promotions while still using the OLD promotion engine. (I won't be using or processing the new promotions yet - I'll only be setting them up).
Is it safe to add my own custom PromotionData / PromotionDataProcessor classes while my site is still using the OLD promotion system?
Is it safe to setup NEW promotions in the Marketing UI while my site is still using the OLD promotion system?
That should be totally fine - as the promotion systems work independently so as long as the VNextFeature flag is turn off then you are free to create the promotions/processors as you like. Only the old promotions will be applied.