You can - but it will overrun your existing website
and if you don’t want that to happen, then you should install WP in some DIR
It means that in the installation options from WP (or Softaculous) you should enter the desired dir name, and not leave it empty (empty means that it will be installed on root).
Or use another free subdomain so you don’t have to worry if it will overrun the existing website
Add user to DB or WP?
DB = Creating additional database users is not possible on free hosting, only on premium hosting. Every account also has one database user. This user has full access to access and modify the tables and the contents of the databases in your account. It does not have ALL PRIVILEGES, but it has enough privileges for normal website operation.
There is no way to create additional database users, or change their permissions, those features are only available with premium hosting
WP = add a user to your WP, but of course you don’t give him admin access, but lower than yours