I actually have never heard of negative matter before and so this was kind of fun and kind of hard, but I will give it my best shot.
First off negative matter in a sci-fi story could be both good or bad, depending on how well you convey it in your story.
Negative matter is completely hypothetical. The debate on whether it exists or not is still on going. But could it exist in our universe? According to this article from Cornell University (PDF), yes, under certain circumstances yes it can. Below is the conclusions of the article.You can also find that article on this page. https://arxiv.org/abs/1407.1457
"Conclusions- We have shown that there exist very physical
configurations of an ideal fluid which give rise to solutions of the
Einstein equations that correspond asymptotically to negative mass
Schwarzschild-de Sitter space times. The energy-momentum tensor that
gives rise to such space times is perfectly physical, it everywhere
satisfies the dominant energy condition. Since the space time is not
asymptotically flat, we evade the positive energy theorems which
would not allow for negative mass. Negative mass configurations
therefore can exist in de Sitter backgrounds, exactly as have been
proposed for the inflationary phase of the early universe. If a
mechanism for production of pairs of particles with positive and
negative mass can be determined, in the early universe there would be
a plasma of positive and negative mass particles. Such a plasma would
in principle cause an effective screening of gravitational waves,
being essentially opaque for frequencies below the plasma frequency."
Here is a link to a page that has a piece of an article that will give you a start on how negative matter can be used as fuel. Written by Robert L.Forward an American physicist and science fiction writer.
In said link he describes how negative matter does not violate the laws of conservation. Also on the wiki page under Runaway motion states negative matter does not violate the laws of conservation.
"This behavior is completely inconsistent with a common-sense approach
and the expected behavior of 'normal' matter; but is completely
mathematically consistent and introduces no violation of conservation
of momentum or energy."
In conclusion, it would be extremely difficult, but possible to keep a lot negative matter contained as well as in plasma form because it is theorized to pass through other particles of mass, completely absorb normal mass and become nothing, travel backwards through time and space, and defy several other laws of physics, which is why many scientist disagree on the existence of negative matter.