The Tyranids will be incredibly needed to ensure that they aren't OP. They will start off very small and build up. I have always had a slight lean towards weak characters (Banha for example, who for all intents and purposes, useless in a world of people who can destroy him in one hit). 

The reason I created Das Jotunheimen is to have an organisation with no sense of good or evil and is entirely neutral. It really depends on the character. The Zombie Duo and Ghazghkull would probably just cause chaos because they hate working with other people. Nathaniel may side with the good guys but it all depends. They are all going to be slightly weaker than they were before by the way, due to some convoluted reason that I'll make up.

If the Tyranids really are a problem for people, *I am flexible and can make sure that I won't be the one Rp'ing them.

EDIT: that last sentence sounds too weird XD. *I can get rid of them