In general it is up to the landlord to request extra security when they have concerns about a tenant's past history and ability to pay. Often, they will accept an additional month's rent as an extra deposit (what is commonly called first and last month's rent) to ensure that they won't have problems getting paid their due rent.
Really depends on the lease situation. If it's a hot property, a full year might get you past the score. If you are the only applicant, there is a little more freedom. And what's the issue with credit? You will need to explain regardless of what you pay in advance.