This is a very complicated process that you are going about performing here... 99.99% of the time you can not just "plug and play" so to speak with new style sheets. For example it is likely that your two inputs up there have float: left
defined in the old stylesheet. The new stylesheet overwrote those styles and made them essentially float: none
which broke the layout.
So, what should you do.
Duplicate the current site out to a development directory (just make a folder in the root directory called dev and duplicate the entire site inside that directory. You'll use this directory to develop from without modifying a site that is currently live.
Include your foundation stylesheet/s below the current one. Then slowly go through the old stylesheet and figure out whats being over-written and merge it to a new stylesheet.