Yes, you must wipe the new computer completely, then clone your old computer or do as you suggested. Since the two computers can be connected by a high-speed TB cable, you can boot the new computer into Target Disk Mode: How to Boot and Use a Mac in Target Disk Mode, then boot the old computer, normally. You should now see the new computer's drive mounted on your old computer's Desktop. Open Disk Utility to do the following:
Drive Preparation - El Capitan or Later
- After Disk Utility loads select the new computer's drive (out-dented entry with the mfgr.'s ID and size) from the side list.
- Click on the Erase button in the Disk Utility toolbar. A panel should drop down.
- In the drop down panel set the partition scheme to GUID. Set the Format type to Mac OS Extended (Journaled.)
- Click on the Apply button and click on the Done button when it is activated.
- Quit Disk Utility.
Clone El Capitan and Later using Disk Utility
- Restart the computer. Immediately, at or before the chime press and hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
- Select Disk Utility from the Utility Menu, then click on the Continue button.
- Select the New computer's volume (named Untitled) from the left side list.
- Select Restore from the Edit menu of Disk Utility. A panel will drop down.
- Select the Old computer's Startup volume (usually, named Macintosh HD) from the dropdown menu in the panel.
- Double-check that you got it right, then click on the Restore button.
The cloning will take a long time because Disk Utility performs a byte-by-byte copy of the entire drive. Note, the drive in the new computer must be the same size or larger capacity than the one on the old computer or the above cloning will be refused by Disk Utility. Let me know if that is the case, so I can give you another option.
