Irule for redirect when all pool members down, need one for each pool? and what is the standard practice for sorry server?

I would say that there is no best practice. There is only "what best fits your situation".

I use a combination of redirects for custom content vanity splash and sorry pages and iRules.

Here is a basic example:

when HTTP_RESPONSE {
HTTP::respond 200 content "Site unavailable



System Maintenance
NOTICE: This site is down for maintenance.
"
}