1 - yes, you need to create a db repdef/sub pair for each replicated db
2 - you can install another repserver if you want, or you can use your current repserver; just depends on your preference and whether or not the current repserver can handle the additional cpu/memory load
3 - when you give the repserver more work to do you obviously (?) affect performance to some degree; the more important question is whether or not your current repserver is at/near its performance limits
4 - did you follow that thread link I provided? did you read Luc's post (within that thread)? if not, I suggest you go read Luc's post for a different way to setup your replication environment; the fact that you state you're not concerned about the switch leads me to believe that a warm standby solution may be overkill (ie, if you never plan on doing a switch then the WS setup is overkill)