Can these stats be applied to NPC as well for dialogue and other interactions?

Also in the VisuStella numbering system where should be keep this plugin?

If you mean can you check if a stat or a skill is within a specific level to control the flow of the dialogue then yes, this is possible.

In a conditonal branch you can use something like this

For skills:
 $sl.getSkillLevel(1, "melee") >= 30
which will make sure the melee skill is above or equal to 30 before allowing specific dialogue or actions to perform from an npc.

For stats:
$sl.getStatLevel("str") >= 10
This will check the first actor's strength stat and if it's above 10 then it will proceed.

We hope this helps.

Sorry, I mean how do you defined the extra stats for the NPC and party members, I might of missed this in my read through of the Documentation.

Like in Dialogue your character has a Charisma of 10 and the NPC has intelligence of 12. Is there a way to put all those stats in there.

I was guessing this is an add on to the stats in RPG Maker.