In order to switch to another country's App Store, you need to provide a valid address and valid payment information for that country. You can always remove the payment information immediately after adding it to the App Store, but it is required when setting up an Apple ID or switching countries.
You will not be able to download an app that was purchased/downloaded from another country's App Store unless your Apple ID is tied to that country. So you would have to download iMovie while tied to the South African App Store. To prevent any legal issues with Apple's App Store agreement, it may be better to have an Apple ID for the South African store and one for the US. If you switch once to download iMovie, and then switch back, you should be fine. They only look into country switching if it happens often and is used to purchase apps for less than in your current country. Like I said, you should be fine since you are simply redownloading an app that has already been paid for.
The issue with having multiple Apple IDs is that you would have to switch between the two if you ever needed to download iMovie again. It's probably best to download it once and then just keep it on the computer. Then switch to the US store and only use the US store. It will look sketchy if you are switching to another country to download apps. If you think you will need to switch a lot, or have a bunch of apps in the SA store, it is probably better to have one SA Apple ID and one US Apple ID.
You may want to contact the Apple ID support team to get an official answer from Apple.
iTunes - Official Apple Support