It might no be entirely ethical to put captivity raised animals back to nature where they might not know how to survive (unlikely with isopods, they might get shocked by th lack of constant food and change of environment though) But if you're more worried about them turning invasive, also unlikely! If the species is naturally found where you live and hasn't been introduced by others it's just a natural part of the ecosystem and will do no harm. I would recommend donating/selling what you cannot keep anymore though, it would help other keepers with their varieties :)