"1/2 as effective against X" Limitation and Player Controlled Allies

[Anaraxes]

You can certainly make up a single name for a category and give it a value so that you can reuse it. malloyd outlined the procedure to estimating the value the new Limitation ought to have. An ability that does half damage half of the time would have full price for half of the levels, and a (say) -20% Accessibility limitation on half of the levels, which works out to -10% for the full price of all the levels. Once you're happy with the value, just name it and apply it to other abilities. You don't have to actually work the procedure for each individual ability.

Note that combining several categories into one Accessibility isn't done by adding the separate limitations, because the Accessibility scale isn't linear. You'd add the probabilities of being useful first, then check that result on the Accessibility scale.

An ability that does double damage versus a category is the same thing as an ability with twice the damage, but with half of the levels limited as "Only Versus Category". Just switch your point of view around from "double damage versus category" to "half damage versus not category".

As for the second question, it's common enough to games to have campaign advantages that are simply assigned to all players without paying for them. You can either ignore the cost, or just bump the campaign starting total to have extra points to cover the required campaign advantages, to suit your taste. CP totals are a relative measure, so either way works.