Hello duby55.
Welcome to the Apple Support Communities!
Congratulations on your purchase of the iPhone SE. If you were not told about the system requirements for your computer to work with the latest operating system on your iPhone, you do have the option to return your iPhone to the place of purchase for a refund (refer to the store's return policy). However, OS X El Capitan version 10.11.6 and iTunes 12.5.1 are needed to work hand-in-hand with iOS 10 on your new iPhone SE. And macOS Sierra is just around the corner.
One thing you can try is to create a partition on your MacBook Pro and install El Capitan on the partition. That way, you can see how the operating system works before converting your entire system.
Be sure to back up your computer before making any significant changes to it.
Use Time Machine to back up or restore your Mac
Disk Utility 12.x: Partition a disk
If you want to partition a disk that you’ll use to start up your Mac, follow these steps.
Partitioning a disk erases all the files on the disk. Be sure to copy any files you want to keep to another disk before you partition the disk.
You can specify the number of partitions and the size of each one. If you want to use one partition as your Mac OS X startup disk, be sure it’s large enough to install the Mac OS X software and any applications and files you’ll use.
- If you’re partitioning an external disk, make sure it’s connected to your computer, and then go to step 5.
If you’re partitioning the disk you used to start up your Mac, print this page so you can refer to it later. To print this page, choose Print from the Action pop-up menu (looks like a gear).
- Choose Apple menu > Restart. Hold down the Command (⌘) and R keys as your computer restarts.
When you see a white screen with an Apple logo in the middle, you can release the keys.
- In the Mac OS X Utilities window, select Disk Utility, and then click Continue.
- Select the disk that you want to partition and click Partition.
- Choose the number of partitions from the Volume Scheme pop-up menu.
- Click each partition and type a name for it, choose a format, and type a size. You can also drag the divider between the partitions to change their sizes. If a partition’s name has an asterisk beside it, it’s shown larger than its actual size in order to display its name clearly.
- If you’ll be using a partition as a Mac OS X startup disk, click Options, and choose the the GUID partition scheme.
- Click Apply.
After you partition a disk, a disk icon for each partition appears in Disk Utility, and in the sidebar of Finder windows.
So, duby55, the decision is totally up to you. We are here to help with whatever decision you make.
Take care.
