Whitelist certain traffic in an existing irule

You don't need to check for the TRACE method in the iRule. You can simply create a custom HTTP profile and remove TRACE (and any other methods you don't want to allow) from the Known Methods list. That way your iRule deals with just the pool selection and all TRACE requests are disallowed, no matter where they originate.

With respect to validating the IP addresses, yes, a single datagroup of whitelisted IP addresses (or address ranges) is the way to go. If an HTTP request arrives from an IP address not in the whitelist datagroup , it can be rejected. So perhaps something like the following:

when HTTP_REQUEST {
     if { [HTTP::host] equals "website.com" and [class match [IP::client_addr] equals IP_WHITELIST] } {
          pool website.com443
     } elseif { [HTTP::host] equals "website2.com" } {
          pool website2.com443
     }
}

With respect to whether or not this will interfere with other sites, I'm not quite sure what you mean. In the state above, only traffic to website.com and website2.com are allowed. Requests for other hosts, such as website3.com will fail. If there are other hosts you need to support with this one virtual server/iRule combo, then you can add an "else" clause to the "if" statement to select the pool for all other traffic, or other "elseif" clauses to select the appropriate pool by hostname, as needed.