In practice, I would say a particle is anything that can be treated as a single, bound, small object. This means that whether an object is considered a particle depends on how you're working with it.
For example, an atomic nucleus is a particle if you're doing spectroscopy, because the structure of the nucleus is irrelevant to the changes in energy levels of the electrons and so you can basically consider it a charged point. But that same nucleus would not be considered a particle in an ion-ion collider, because the structure of the nucleus is highly relevant there.
More generally: quarks, gluons, protons, neutrons, atoms, molecules, cells, grains of dust, raindrops, baseballs, satellites, planets, stars, and even galaxies can all be considered particles in an appropriate context.
It doesn't fit the definition of a particle
This is an example of why you should never trust a general-purpose dictionary to give the appropriate definition for a term used in a technical field like physics.
I have been told on multiple cases that a particle is the smallest form of composite matter.
That's not necessarily true; as I've said, it's context-dependent. However, that is the definition implied in the term "particle physics" (which is really meant to mean something like "fundamental particle physics").
You missed a rather important conservation law:
$$m_nc^2 < m_pc^2 + m_\pi c^2$$
But in general, yes, the only way to really confirm that a reaction is allowed is to check all the conservation laws. This is why we have tables of decay modes. Other people have checked the conservation laws (and done experiments to back it up) so you don't have to. I mean, it's still an important educational exercise, but in the "real world" when you just want to know if a particle decays in a certain way, you can simply check the tables.
Best Answer
spin 1/2 fermions (electron, proton, neutron, muon, tau, quarks) have +1 parity (by convention as pointed out in Anna's comment). The corresponding anti-fermions have -1 parity.
Bosons and their anti-particles have the same parity.
See this and this lecture for more information on parity.