Using a second HD and installing apps on external drive?
Mac apps run pretty much anywhere you care to store them.

Unfortunately, most Mac apps are also so small that the time you spend moving them off the boot drive doesn't compensate for the amount of space you recover.

You might look to store things like video and then photos and last music off the main volume as they typically take far more space.

As long as you don't have two copies of an app, you are free to quit any app and move it on or off the main volume as you please so other than losing some time, you can start now and see if this plan helps you reach your storage goals or if you need to reexamine them once you've moved all the apps you care to relocate.