1. You can ONLY withdraw from the 401k account associated with the company you are leaving between 55 and 59.5. So it is a good idea to roll any previous 401k accounts into your current one before the age of 55 to give you that option.
2. There are no penalties or additonal taxes for withdrawing from that account at 55 as long as you leave the company. It is simply taxed as income and is no different than withdrawing at 59.5 from any 401k account.

CPA's correct me if I am wrong.