You mean on one MySQL database and not server?
It's not recommended to install many scripts on one database. This will bring no problems if they don't need a table with the same name, but it can make access to it slow, which will reflect in the time your website takes to load.
It's better lo leave the database clean, with only information needed. This is also a reason why scripts should be installed in different databases, if you don't need a script anymore you just have to delete the database. You can delete the junk information on your database (it's recommended), but you've to make sure you're not deleting something you'll use in a script you're actually running.
MySQL databases space also counts on your DiskSpace.