doesnt matter if you dont have immobiliser mate. the donor cars ecu still has to be unlocked ask the scrappers if they have a key for that car (if it has a running engine) and start it up and take the immobiliser out of that one, this will unlock that ecu and then it will work fine. if you cant get the key however as mentioned above get onto username "cam" he does it for £40. send him the ecu he will unlock it and he will send it back.
it doesnt matter if the ecu is from same engine type as yours, if that ecu hasnt been unlocked it wont work with your car. even with no immobiliser you still have a transponder in the ignition barrel that wont recognise the donors cars ecu.
your only other option is get the ecu,immobiliser and igntion barrel out of the same car and that will work fine