I think the holding pen and strip search would prevent any zombies from sneaking in. There is a audio interview and a secondary screening/strip search. The zombies couldn't make it past that one since they still do not appear to have mastered speech and look pretty conspicuous up close. Remember that since Hope is at the Colony, other survivors from that Tower would be there and the Colony would have learned from the downfall of the first Tower.

I don't know how the Colony would stack up against a Maller infiltration. It seems that once you prove you are not a zombie you are free to roam around with your gun.