To skip slow tests (except when you want them), you put them into separate targets and then don't invoke them on the command line (except when you want them). At the risk of tossing you in the deep end with a link to a big build config file, https://github.com/pantsbuild/pants/blob/master/tests/python... shows some Python tests that have been grouped into test suites. One of those test suites is named "integration". When folks want to run the fast tests, they invoke on tests/python/pants_test/targets target; when they want the slow tests, they invoke on tests/python/pants_test/targets:integration.
Yes, you can add a goal to generate compressed CSS. The docs are, alas, still skimpy, but they're getting better; meanwhile, they give an inkling: http://pantsbuild.github.io/howto_customize.html
Maybe, maybe not. As your codebase grows, it's likely to turn into separate parts with some common code. Pants can get you faster incremental builds: quickly figuring out what needs building, building just that. If sbt is quick for you, you might not have hit the point where Pants helps.
If you reach a point where sbt gets slow and you're looking at other build tools, that's a good time to look at Pants.
When you invoke Pants, you pass in "goals" (roughly, the verbs/actions you want to take) and "targets" (roughly, the things you want to act upon, often piles of source code).
If you usually invoke './pants goal test bundle src/my/awesome/project tests/my/awesome/project' (to run tests and bundle up an app), to skip testing instead invoke './pants goal bundle my/awesome/project'.
The word "goal" should disappear from the command line at some point. Alas, we're not quite there yet.