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The Manfrotto video plates aren't compatible with their camera ball heads. Personally, I really don't like their ball heads or (camera) plate system.
For a good ball head for decent money, I'd look at Sunwayfoto or Sirui. I've reviewed a couple heads for Sunwayfoto, and been very impressed with their quality, even their entry heads. The Sirui heads seem a little more consumer-grade in their build, but are even more affordable.
I don't know anything about mounting for Steadycam, but I'd consider Arca-Swiss if you're using an SLR in that setup and want compatibility with regular head/tripod systems.