iRule to forward traffic depending on specific IP to original pool when holding pool enabled.

I would probably go with something like this.

when CLIENT_ACCEPTED { 
if { [class match [IP::remote_addr] equals external_ip] } { 
    pool  
    }
else { pool holding_pool }
 }

I don't see why you would need seven different holding pools, but if you do then you'd have to get more complicated to rotate through those pools. I think it would be easier to combine all the "holding" servers into one pool.