Bucon

Yes it can work. No it's not suggested.   I have run a quick bench test and a fx2n-8ey works with no issues. However, a 16ey seems to miss some outputs. I would suggest this is the same for the ex. Sadly I don't have any ex units to try as they are quite outdated now. Hope this helps anyway.

how are you driving the master servo? (ie, pulse/digital run eta) depending on that i have a few ways, one that comes to mind is feeding the encoder pulses from the master to the slave's pulse train.

specific is io relating to system bits that are ether set by or to the controllers internal system (IE. Servo on, estop, alarm, start, pause...) general io is user defined io, these are used freely as required and are accessed directly by the robot program in most common cases.(in0,in1,out1,out2...)   external io is a special number given to all io regardless if its specific or general, think of these as hardwired numbering of all Digital io. this is necessary as for example I can change the input servo on to another input. this is more advanced and only done when more familiar with these systems.   hope this helps a little.