For those questions I would refer to the http://wireless.att.com site. I currently have AT&T and I believe mine start at 9PM, but you can upgrade to7PM with an additonal monthly fee.
I believe roaming in the US is free on ATT. It's a nationwide network. To the extent there are only non-ATT GSM towers in the area, that roaming is free (not sure how many such areas there are).