You will probably need to install a lot of mods for those crafts to work, but if you still want me do it just tell me. Also, yes, the decouplers are just above the probe core, check the last 3 images zoomed in. If more than one fuel tanks are present and not vertically stacked in a stage , then FMRS will not work.
Edit: Also tell me if a mod list will be helpfull, or a log ( which i don't know how to get yet ^^)
Edit 2: The stage continuous to fire because FMRS didn't detect the seperation to cut the engines off