I'm not sure if Ignition stacks but for the other question, check this thread : https://forum-narutoen.oasgames.com/page/show-post-24555-1.html

ignition does not stack and tag reduces resistance

There are a few small exceptions to ignition stacking, and that's basically any ignition that is applied for multiple rounds, which as far as I know only comes from Founder Madara and Susanoo Itachi chases (3 rds each), and maybe a few others I'm not familiar with. Ignition like this stacks with the more normal 1 round ignition, but not with each other as far I've been able to determine. So at most only 2 stacks of ignition, and I have confirmed this in PvE extreme team exam (4k off ignition on elemental puppets is a life-saver :P).

Tag also reduces Def as well.

That's why Minato and Konan, They do a lot of damage.

Ignite does not stack and it does a max of like 1999 damage based on overall hp. Tag reduces defenses and resistance by 20%.

It doesn't stack. I use itachi susano'o a lot in 3vs3 arena. I have never seen a 2k+ ignition damage, regardless who i or my pals use as other ninjas.

@OP: ignition doesn't stack because as a secondary effect makes healings useless if stacked would be stronger than poison and this would make no sense.

Tag reduces the enemy defense and resistance by 30%. Beware that this doesn't mean your damage will increase by 30%, it's up on how high those values are on your enemy. The higher they are the lower the effect of tag is due to diminishing return of def/res (in the sense the difference in damage dealt to a target with 7k res instead of 10k res is way higher than the difference between the damage delivered to a target whose def was brought from 20k to 14k).

Perhaps it is a PvE only scenario because I have for a fact seen 4k ignition damage (only possible by stacking) in extreme team exam.

i tried this lineup and by looking at the hp of kakashi at the end of the round, kakashi actually endured, after the last clone hit him, 11994 damage per round after round 4 (4 stacks of poison are 7996 so the other 3998 had to necessarily be ignition, so it actually stacks as you say, but not directly, just repeat twice 1999 instead of writing 3998).

The damage from 1 "layer" of ignition is exactly 1999 (if your nin status is higher than your opponent resistance, otherwise it will reduce) and it stacks if the number of turns applied are different (for example a 3 turns Itachi ignition and 1 turn roshi).

Oh my, my fault then for never noticing this.

throw in toxic or strong poison and we have a dot of 13993 I think hahahahahha