I would recommend searching the forum about solid state hard drives and their limitations. 

Legacy content in PP2014 will contain the same content found in P7 and P9, you will not need to transfer everything.  The only drawback will be additional downloaded content you have chosen to place into those runtimes.  I would not advise combining earlier version runtimes with current simply because you will trash most Python Scripts.  It would be better linking them as external runtimes then picking through them carefully to eliminate duplicates. 

Remember, you are better off creating smaller, dedicated external runtimes than enormous ones.