Should be no problem.
You just can't mix a down stream 1st with and up stream 2nd, i.e.- a poseiden 1st with an Apeks 2nd.
Someone else will have to chime in on the unbalanced 1st with the balanced 2nd. I suspect again no worries because the 1st is going to provide the largest change in that config.
Not quite, but its the right idea.
Some poseiden second stages use what amounts to an expandable collar to control gas flow, so they are not "upstream" second stages per se, but they are not "downstream" second stages in the sense that they will not function as a pressure relief valve in the event of a high pressure seat leak in the first stage.
Diaphragm first stages, regardless of brand are "upstream" designs as the seat and seat carrier are located upstream of the orifice and the tank pressure acts to hold the seat on the orifice. Piston first stages are "downstream" designs as the seat is located downstream of the orifice and tank pressure acts to push the seat off the orifice.
When balanced it makes no real difference as whether the tank pressure acts to open or close the valve does not matter. No one makes an unbalanced diaphragm first stage but in the past, those regs got easier to breathe as tank pressure dropped as the IP increased with lower tank pressure. With unbalanced first stages, the IP drecreases as tank pressure falls and with an unbalanced second stage, inhalation effort increases.
Unbalanced second stages use a spring tooffset the downstream force from the first stage. If the first stage seat leaks, the pressure increases and this excess pressure will force the second stage open and the resulting leak will serve as an overpressure relief. Balanced second stages use a combination of spring and air pressure in a balance chamber to offset the downstream froce from the first stage. In this case, if the seat leaks, the pressure onboth sides of the second stage valve poppet increases. This could be bad news, except the valve is designed with a downstream bias so that it will still function as a pressure relief to vent the excess pressure before a hose ruptures.
What it all means is that the only time it matters is if you attach a poseiden second stage (a non-downstream design) to any first stage without either a poseiden hose with a built in OPV or an add on OPV screwed into one of the LP ports on the first stage. The second issue with poseiden first and second stages is that some of them operate at a higher IP in the 175-190 psi range, while everyone else operates around 120-145 psi.
