Ok, everybody calm down, I am alright!! I do not have crawfish, everywhere, YET??

I do have a question though. I have not considered putting crawfish in my system, because I have read and have been told there is not way to filter the things out and keep them isolated to one tank in the system? I have heard that their offspring are so small they will bypass any filter and it is inevitable they will end up in every tanks and every part of your system. Now, because I am considering a DWR system, I do not want them in my troughs eating all of the plant roots.
So lets hear it from those of you who have "been there done that". Is this a myth, an internet fairytale, some evil untruth devised to keep those of us in AP away from the tasty little crustations?? Or is it true that they eventually will inhabit every part of your system and eat all of the plant roots, generally causing havoc???

O, I almost forgot I am originally from Louisiana and they are Crawfish, never even heard the term Crayfish, till I moved to Arkansas. Crayfish, that just sounds funny!!!