If you are making offer on short sale then that means they are in agreement with the transaction so there should be no redemption period. I am not a lawyer but that is how it works here in California. Yes, if they do not move you make be forced to evict them but that is mostly happening on foreclosures, not many short sales. If they appear cooperative then odds are good that you will be fine. It is always a risk but one that is almost everywhere in this market.
Once you own the property it is your right to remove by legal means anyone that remains in the property. Check with your RealtorÂ® about doing a final walk through and I hope in the contract that you have specified that the property is to be delivered vacant.