You already answered yourself.
"ECU is from a different car"
Your ECU has one SKC number...
"cluster is also from a different car(different from ECU)"
Your cluster has another SKC number...
"when i try to login using the same SKC for instruments it says the code is incorrect"
There is no point trying to use the ECU SKC in a cluster from another car. This cluster have another SKC, you must read it to use it.
"ECU is from a different car and has Immo turned off"
You have to address this too. Immo must be on for proper ECU/cluster adaptation.