hello aurelio,
i like your ideas and design of heating your room with different systems.
you can combine one or more heaters to one flue. no problem. only one thing..... you have to provide them with air tight dampers for each one of them.
you even can burn heaters at the same time, but only if second one is a closed door system. (no openfire like rocket or batchbox , but a normal woodstove with door with air intake regulation)
of course, everything depends on how well your flue is working. (stone, inox, isolated, more than 5 meters long, highest point on roof, ect, ect.)
it is very practical to have a cleanout in the cellar, or a flue warming up for cold start. see no problems there too.
also the bench with 3 pipes will work! but not all 3 at same time. you will need direct one of them at the time with 3 (not so air tight) valves. so you can experiment with the best settings.
but im not sure if the extra build complexity will bring you more efficiency or joy for that matter.
i have experience for years with batchbox, bell, mass benches.
there is simply no 1 perfect solution in one system.
at the moment i use my rocket the most, because of my (cheap) wood supply. i don't like sawing wood. so i use long/ small sticks. (even 2 meters long)
another reason is because it burns better at long burning times with less heat. (with sticks and cheap wood findings alongside the road)
in the morning i need quick convected air warmth from the steel rocket barrel (before going to work).
in the evening a slow, more stone mass heating fire is required (for the night.)
for less cold days, of course only one fire is required. (cold days i will use 6kg wood. 25m2 kitchen, not well isolated)
the batch box system or normal stove i like to use when i'm not around to fiddle with fire. but uses expensive wood. (in money or workout)
a bell is nice, quick build, but also loses its warmth easily to the room.
the mass of a bench is more "warmth durable". you can isolate it on top with wool cushions and regulate in that way your warmth release to the room. (in night you will need less air to heat.)
i came up with a bench which is a combination of a thin wall bell front and mass back.
the front is a kind of bell/ syphon system, the backside of the bench is with a normal flue.
short fire, bell does it's work.
it will warm up to 60 or even 80 degrees (celcius) and helps convection (of the barrel) warmth for one day.
the back takes a longer fire to warm up completely.
the back will be 40-50 degrees max. and is convenient to keep the room at 15 degrees 24 hour round.
so in designing them you need to stay open to experiment and learn from your mistakes. in other words, if you want "one build", a near perfect system in terms of guarantee, please stick to the plan of known working systems, like peter advices you.
