For #1: You could try the Fixture Sets plugin, although I haven’t needed this personally. One set is enough to worry about, but to each his own: http://thatswhatimtalkingabout.org/news/2006/8/31/fixture-sets-for-rails
For #2: Fixtures are evaluated with ERB. Just put some ruby between <% %> and it will execute.
For #3: Try the Fixture References plugin: http://wiki.pluginaweek.org/Fixture_references
Also if you want to dump your existing database tables to yaml fixtures, try the AR Fixtures plugin - http://agilewebdevelopment.com/plugins/ar_fixtures
