just for clearification, the Solid Edge term you are looking for is "Adjustable" not "Flexable"
RMB the Sub from the PF and make Adjustable. this will allow control of any underconstrainded relations from teh sub a tthe highter level. there is also the ability to Promote and sub assy constraing relationship to the upper level and control that from teh upper level.
If the subassembly in question is something that is always going to be adjustable when placed into a higher level assembly (as I would assume your example of a pneumatic cylinder would be), there is something you can do to save yourself some time. First, open the subassembly. Then, go into Solid Edge Options. Under the 'Assembly' options, set the following option:
Setting this option will then make that assembly adjustable every time you place it as a subassembly into a higher level assembly, so you will not need to do it manually. It can save a lot of time.
Hope this helps,
@GregLuckett, is this setting supposed to be recursive when there are several layers of adjustable assemblies?
That selection you pointed out seems to not always work and we go into the lowest adjustable assembly set it adjustable. I haven't dug into why those assemblies with that selected do not always place as adjustable yet.