"We are pursuing the criminals."
To confirm my understanding: Technically HeSwI'pu' is 'criminals' but the plural suffix is optional and in this case, not strictly necessary as the prefix DI- on the verb indicates the plural? Yes?
Technically the plural suffix is almost never strictly necessary, though it does help to clear up the meaning when the verb prefix does not indicate the plural. However, you are right that in this case, the DI- prefix makes it clear that the object must be plural and can't be singular even though it lacks the plural suffix.
More technically, there ARE times when the plural suffix is necessary, but this is not one of those times. Those times are when you want to say all of something, like Hoch tlhInganpu' all of the Klingons. If you said Hoch tlhIngan, it would mean each Klingon, considering them as individuals instead of as a group.
There are also times when plural suffixes are not allowed, even when you're talking about a plural. One of those is the each of something meaning above. Another is when using the word 'ar: a noun followed by 'ar may never take a plural suffix. So to say how many Klingons? you must say tlhIngan 'ar and may not say the grammatically incorrect tlhInganpu' 'ar.
But MOST of the time the plural suffix is optional. In my opinion it is best used only when the plurality of the word is both ambiguous AND important. reH Suv tlhIngan might mean A Klingon always fights or Klingons always fight, and it really doesn't matter which one it is. The speaker doesn't even need to have one in mind.
Plurals are helpful for the beginner, but you should get used to the suffix-dropping you'll find "out in the wild."