[dspillett]

The way I've explained the correctness of the plural use to a non-native speaker, is that (to use the cats example) there is one group of cats, the group is singular even though its content is plural, so “there is” is correct, therefore so is the contraction. Not explicitly mentioning the group is just another abbreviation, because it is implicitly understood. This avoids the discussion of whether “there are → there's” is valid in written form (which tends to be more strict than verbal use).