I'm working on a new October site. I decided to use the SQLite to see how it would perform. I'm using an events plugin and might have to use MySql with that. Is it possible to just use MySql for the plugin and continue with SQLite for the CMS?

Yes it is! Models use default connection, in your case it is sqlite, you should add mysql connection information in your config/database.php file and then manually set public $connection = 'mysql'; in every model that will use mysql.

in /config/database.php define your sqlite and mysql connection.

Then in the models where you wish to use one or the other, you can use the property $connection = "connectionname"

class MyModel extends Model {
    public $connection = 'sqlite';
}