I noticed there's a similar answered question here, but that one's pertaining to the me package. I'd like to know if and how, if so, is it possible to that using ms instead.
Well this is embarrassing! A short while after giving up and posted this question I stumbled upon the answer:
.ds FAM H
If it's the first line in the document it'll set all text to use Helvetica by default, requiring you to use \f[font family descriptor] to change it to e.g. Times Bold-Italics. If it comes after the cover page, that page'll be in Times and the rest of the document'll be in Helvetica.
Hope this helps anyone who has the same problem I had :)