You can’t go wrong with Oceanic regs, reliable and can be serviced pretty much anywhere. I have a few and the one on my stage bottle is a Delta. I never owned Apeks but many other brands have been private labeling them as their own including I’ve heard ScubaPro in the past.
"types of gas mixes it is compliant with " Just about any reg except Atomic Ti’s can be serviced with o2 compliant greese and o-rings to make them o2 safe. Don’t forget the tanks need to be o2 clean as well.
Just one caution on the Poseidon regs, they use a different IP presure than other brands so a second on your first stage would also need to be Poseidon.