The only thing you can't do is use the 200MB free in anything other than the device that the 200MB plan was activated in. As far as paid data plans go, you can use as many on one device as you want.
I found out about the 200MB free being limited to the device it was activated on when I went to use a sim card that was activated on a Nexus 7, and put it in my iPad. the 200MB wouldn't work and so I couldn't use my 3GB data plan either because that wont get used until the 200MB free is used.
I had to call T-mobile and have them remove the 200MB free so that I could use my 3GB. They told me in order to get the 200MB free for my iPad, I would have to order a new sim card. That was about 6 months ago and I don't need 200MB free so i am still using the same sim card.
There is nothing stopping you from having as many sim cards that use the 200MB free on one device. I once had four that I was using on my iPad. All four of those SIM cards quit working with the 200MB free when I got a new iPad. I would have to get new SIM cards activated on the new iPad in order to get 200MB free again.