No, small plants make just as nice a bud as larger ones. the buds are just smaller. A 400 will do the job easily. Yes, use MH for veg, and HPS for flowering. As long as the plants get fresh air, in a space that small, it doesn't really matter if you're pulling the air or pushing it, as long as it's moving it. Heat may be an issue, so plan for some kind of way to control temps. Indica dominant plants generally grow shorter and bushier, so look for Indica dominant strains to grow.