Peter
I would second what you have already been told. I would throw away all the pipes, joiners and valves you get and replace them with Robart or BVM gear and pipe. The retract units will have to be removed from the plane and tested one at a time with soapy water to make sure that are no leaks either at the connections or the inner and outer o rings. Most modern planes come with retract cylinders that are really too small. This means having to use lots of psi and higher pressures mean more leaks. I had a Fei Bao Su27 which had nice looking retracts that were very marginal and needed 120 psi to retract in the air. If the retracts are screwed together all screws should be taken out and assembled with Loctite. Mine came apart on the flying field after 2 flights on grass.
I prefer to use electronic valves but they too need checking for leaks. The larger the amount of air in the tanks you carry the more retract cycles you will get. You should aim for at least 4 cycles and that may need 3 large cylinders. You should not need to worry about safety devices if the gear holds air and is well checked out. It always takes a lot less pressure to drop the gear than raise it. Dont forget if there are gear doors attached to the legs they will catch the wind and act to stop the gear going up. I always use a BVM filler valve but even these have been known to leak, so check everything. The gear should hold air for several hours.
The doors I always make a separate system for as they do not need as much air pressure as the gear. My FB door rams worked except for 2 that were a pain to reach and I replaced with UP rams. The doors need to operate faultlessly as they can hang a wheel up.
I use RCBee sequencers which has a system that makes it impossible to switch on the Tx and retract the gear by mistake I have had several and never had a problem with them.
John