I'll pick up one of your questions, because it was an issue I had starting out.
Do worry about circulation (flow) in the tank. You want to avoid "dead spots." Good circulation helps keep oxygen levels and other parameters stable around the tank, which is one step towards encouraging healthy plant growth. Cannister filters provide great filtration, but once loaded up with filter media, may not provide enough circulation. A powerhead can supplement any additional circulation a cannister is not providing.