Learn English – When should noun modifiers be plural and when singular

Persons is very rare in normal English speech. Mostly you only come across it in legal or other "official" contexts such as...

The defendant conspired with a person or persons unknown to blow up the House of Lords.
6 persons maximum/Licensed to carry 4 persons (notices on lifts/taxis).

In most normal contexts the plural of person is people. When making a restaurant booking, for example, you'd normally ask for a table for six people - if you said six persons that would suggest you're nervous, unfamiliar with such situations, and foolishly trying to sound "correct" in an inappropriate context. If it was a swanky restaurant they might just say they're fully booked because you sounded gauche.

Individuals is also relatively uncommon in speech, tending again to be restricted to official (particularly, written) contexts. Probably because of this, if you said you saw four individuals somewhere, it might well imply four suspicious-looking characters, since the phrasing is typical of witnesses giving evidence in court, rather than everyday conversation.

Note that individuals carries no connotations of each individual being significantly different to every other. Identical twins wearing similar clothes are still two individuals, if the context permits using the term at all.

There's more on this subject in Person, Persons, People, Peoples, which was asked previously on ELU, but for most purposes I suggest it's enough to note that the standard forms are person/people.

The gerund refers to the act or process of doing something - the activity itself and nothing further.

The -ion form of a root can mean the act of doing something, but usually leans toward meaning its result, effect, or manifestation - something that persists or evidences after the activity.

Why don't some verbs have "-tion" nouns?

This may be a better question for https://english.stackexchange.com, but briefly looking into it, -ion comes from Latin and is generally used with words of Latin origin. Non-Latin words - such as those that are part of the Germanic "core" of English (e.g. all the irregular verbs) won't work with -ion.