If you purchased your license after the release of SQL 2014 then you are bound by the restrictions that were new to SQL 2014 even if you are 'downgrading' the license to an older version. In particular, the right to have a passive node without any additional licensing only applies if you have SA. Also, most virtualisation scenarios require that you have SA, this came in with SQL 2012.

If you purchased your license prior to the release of SQL 2014 then you will bound by the restrictions that applied at that time.

If you are not certain of what is covered by your license then the people who use this site can give an opinion, but Microsoft is not obliged to take any notice of these opinions. If you want certainty then the only way to get this is to talk to your license reseller.

Also, depending on the terms of your existing agreement with Microsoft, the cost of adding a new SQL license to your estate may be different to what you expect - the only way to be certain is to talk to your license reseller.