They don't need real bright light. A single T5 strip is plenty to get them started, but they will grow faster with more intense light.
You can start them growing with just a few inches of clearance, but as they grow they will need more room overhead to reach up. You can set up a hydroponics strip light as an economical pendant fixture that will work real well.
Hey here is this offer again. These little mangrove trees really are a lot of fun and pretty easy too. They start to root in their planters right away.