I only use certain apps rarely, like a clock sync app, so I don't really need it in the main memory. Can I just move it to the external SD card and launch it only when I need it?
Not sure what you are after. Link2sd will move the app, lib, and cache for the apps to sdcard. It will not move the apps's data which should be minimal.
Link2SD will move the application, but it can still run. Moving to the external SD will totally disconnect it or prevent it from running. The only other option is to uninstall the app, and reinstall it when you need to use it, which is say, once a year.
So is there a way, and it seems there should be, to set the sync to manual, so that it only runs when requested? You want the utility available, but don't want the system or any app to run it on it's own, is that kind of the idea?
Here're two ways for you.
1. Move to SD card directly
Just enter "Settings">"Application Manager", and you will see apps on your phone. Find the destination app, click it and you will see "Move to SD card" option.
2. Transfer to SD card by Android Transfer
Sometimes you will be fail with the first method because of these of those reasons. So I will introduce you a third party app - Android Transfer by RecoveryAndroid. It is a wonderful data manaagement app with safety, convenience as well as powerful functions features. You can run the program on PC, then connect your Android device. The program will detect phone data automatically and show you in categories. You can feel freely to click "App" category and select destination apps. Finally with a click of mouse, you will be able to transfer apps to SD card on Android.