What you want to do can be done.
If migration assistant won't do what you need to do, here's how to "do it by hand".
First, get the SSD installed into the MBP.
Get a BOOTABLE, WORKiNG COPY of the OS installed onto it.
Create an account on it with the username and password that you're going to use.
Do this BEFORE you begin trying to "manually migrate" stuff.
Probably the easiest way to proceed from that point is to manually re-install your non-Apple apps (such as MS Office, etc.).
NOTE: these could be "brought over" from a cloned backup, but that would involve some extra steps.
Once you have your apps "over, setup and running", then it's time to think about data files.
What I would do is to use something like an external drive or a USB flashdrive to serve as your "moving van". With a flashdrive, you might have to make more than one "trip".
I would suggest you set up the external drive as follows:
Create new empty folders on it, such as:
"To documents"
"To music"
"To pictures"
"To library"
etc.
Then, on the iMac, open these respective folders (documents, etc.) and select and copy those items to be moved to their respective folders on the external drive.
When you have your external drive loaded up with everything that needs to get moved, connect it to the MBP and then do this:
1. Let the drive icon mount on the desktop, but don't open it
2. Click ONE TIME on the icon to select it
3. Type "command-i" (eye) to bring up get info
4. In get info, at the bottom, click the lock icon and enter your MBP password
5. In "sharing and permissions" put a checkmark into "ignore ownership on this volume"
6. Close get info
WHY you did this:
Now you can copy anything on the "moving van external drive" to the MBP account, and ALL the copied files will "come under the ownership" of your new account.
Thus, you avoid permissions conflicts.
OK, next thing to do.
Let's say you have files from the "To documents" folder that (naturally) need to go into your documents folder.
You need to open the "documents" folder for your MBP account (have it visible on the desktop.
You need to open another window showing the contents of your "To documents" folder (on the external drive).
Now... just select the files (from the "To documents" folder) and manually move them into the "documents" folder.
NOTE: If you had tried to just move the folder itself, it probably wouldn't work, because the "main sub-folders" in your account are MORE THAN "just folders", and don't behave as do normal folders. That's why you have to move FILES, instead.
NOTE: you can also move folders that are INSIDE the main sub-folders. It's only the "main sub-folders" that you can't move (documents, pictures, movies, music, etc.).
An additional note:
You can also move folders "outside of" your home folder THAT YOU CREATED YOURSELF.
No problems at all doing this. Just "copy away".
Moving things like Mail files can be more complicated.
You have to "go into your home/library folder and start picking and choosing that which you wish to bring over.
Same for application settings and preferences.
This is where it becomes important to keep handwritten notes as you go along.
Actually, my suggestion is to set up a mail account, but just "leave old emails where they are" on the iMac. It's just easier this way.
Sounds like a lot of work, because it is -- IF you wish to migrate very selectively and manually.
My last advice:
It's a fool's errand to try to get the MacBook to be "just like the iMac" insofar as things are set up.
It's just easier and less trouble to "let each Mac BE ITS OWN Mac", so to speak.
At least, that's what I do.
