is this irule ok?!

You were close with your initial attempt. Whenever you're using a logical operator like or, you have to repeat the comparison operator as well. Like this:

 
 when HTTP_REQUEST { 
   if { ([string tolower [HTTP::uri]] starts_with "/subsite") or ([string tolower [HTTP::uri]] starts_with "/subsites")} { 
     pool subsite_pool 
   } else { 
     pool main_pool 
   } 
 } 
 

Realistically though, starts_with /subsite includes starts_with /subsites anyway, so you should only need the one comparison.

So really, this should get you there:

 
 when HTTP_REQUEST { 
   if { [string tolower [HTTP::uri]] starts_with "/subsite"} { 
     pool subsite_pool 
   } else { 
     pool main_pool 
   } 
 } 
 

hth,

Colin