Hi, welcome to LQ
I can't help you that much as I never used ppoe.
To have a broad understanding about how network is usually configured in Slackware you could read
this article written by Eric Hemeleers aka AlienBOB, one of Slackware's main contributors.
AFAIK the netconfig script (which is called during setup at time of install) is only useful to configure ethernet interfaces.
To avoid IPV6 address you can edit /etc/lilo.conf's entry for the kernel you use and add a line with:
append = "ipv6.disable=1", then run lilo.
To change your machine's name edit /etc/HOSTNAME. See also the files /etc/host*
As stated in aforementioned article you can use wicd as well.