The Sioux body grinder wouldn't be much use for shaping foam. For one thing it takes some muscle to handle it and a lot of practice to learn to control it. It really shines for shaping fiberglass or when you are shaping a part with different hardness materials in it. I never used used foam back in the day. I used wood, fiberglass and bondo. Properly handled, it will cut through all three materials evenly. Probably overkill for foam. I was a husky lad when I was building plugs. I am to old and weak to use the 7" grinder anymore. I use a little Makita 4" grinder now. I still have the touch though!
If you do want to try a grinder, this is the way to do it. You use a sweeping motion - like waving your hand. Start by sweeping the grinder back and forth above the part and gently lower it until it just touches. Keep moving it - don't stop! You want to use the outer part of the disc but not the very edge. The disc will flex and grab when it hits and you have to be sensitive to that. You adjust the pressure you are putting on the tool according to the hardness of the material you are grinding and how much you need to remove. You will have to experiment a bit to get good results, but you can do precise work with it after some practice.
Can you be a little more descript with these grinders, with a picture? Are we talking about orbital sanders? Just trying to be on the same sheet of music on this...and also planning ahead for my full scale build!