1) Spot Starting is a not uncommon fantasy baseball strategy. In fact, the Cafe has it's own thread on the matter here
thanks go perlick. My main advice would be not to abuse spot starting, as there comes a point where it becomes 'churning', and you can simply do a search on these forums for that word to see the verbal wars that have been waged over that 'strategy'.
2) The game links on Yahoo as well as MLB.com will have daily rosters posted generally 30 - 60 minutes prior to game time; basically after the manager submits them. Beyond that though, you can only speculate, really. However, understand that you're not going to field a full roster every day. For example, in a standard league, if my Catcher just isn't playing that day, well, I'm not playing a Catcher that day, because a roster spot is MUCH more valuable to me than to waste on a backup catcher, or more than 1-2 backup bats at all for that matter. In a standard 5x5 H2H league, I'd generally fill all my SP slots with starters, all my RP and P slots with closers and/or high K/low ratio relievers, and then stash my bench with more Starters.
I hope I've been some help to you. Good luck.