Can I take out my SSD from my old Mac. Connect with this? To my new Mac book? To move it manually?I think the part you are missing is that to use Migration Assistant to import that data, you have to check the box to import that account, so the permissions associated with that account userID come in also, and that is what can cause problems.
Sure, if you just make a new account during the original setup then attach the backup drive and only drag in say your Photos library, you will not have these permissions issues.