If the smaller pleuro(s) is small enough to be eaten the larger ones will NOT hesitate to swallow them. I also do not know about territorial behavior, I know pleuro's do best in groups but i am unsure if an established adult group , or even solo adult will except new comers or if they need to be raised together.
I would tread VERY carefully with an introduction. First grow them out separately until they are big enough not to get eaten after that all you can do is try and hope!
Personally I wouldn't try it unless your setting up a new tank and planed to introduce both groups to each other at the same time into a new home to eliminate any sort of territorial battles and increase the chances that the groups merge to from a new larger school.
could be risky.. they always bicker and fight between eachother for dominance. I wouldnt be surprised to see one killed if it isnt at least 70 or 80% the size of the largest
I don't think I would put them together; unless they are the same size as if one is bigger the other may eat it :/ even if too big to swallow whole, the bigger one may attack and pick at the other until it is dead... Just my $0.02
My 1 pc pleuro is now (guesstimate) 5 inches. I'm planning to add new ones which are 3.5 to 4 inches.
If you guys are saying that its a bad idea then I guess I'll let it have the tank to him/herself.
Oh at those sizes you should be fine, in fact that sounds like a very good size range, yours are the exact size I'm looking for to piece together a group of my own!
It's sub-adults & adults that are tricky yours are still just juvies. a range of 3-5" is a very small difference and very young fish I wouldn't expect problems.
My comments above were assuming that by older bigger fish you meant something in the adult range 14-16".