A cpanel full backup backs up everything, including garbage files, this is normal don't have to worry too much
as for which sql file to use, it depends what CMS you have hosted before in your old site as well as the usage
remember, you + your host + the CMS named the sql, there is no default name so I can't tell you which one to use
however, a good indication is just use the biggest sql file, if you installed smf multiple times, but you only used one for a long period time
as for the current database name of your forum, open settings.php
find $db_name, that is the database name that your current installation is used
you can also look up the old forum's setting file to determine what database sql it was using