I would have to guess that you would probably want to wait for the tank to finish cycling. Ammonia, from what I understand, is pretty toxic to any organism (except for the nitrifying bacteria).
In my opinion, it is better to be safe than sorry. There's no sense paying all the money for a detrivore kit to only watch it die during the cycling process.
This hobby is expensive enough without having to worry about buying two detrivore kits (the second being to replace the first kit that died).