in Shadowrun Missions they run with this optional rule in effect.
1: what all does it affect?
2: if you buy successes do you buy them off of the total pool or the capped 20?
2a: for example most vehicles will roll more than 20 dice to resist damage, do they only roll 20?
2b: super Dwarf (Body 7+ BoneDenseAug 4) in Form fitting Armor and an Armored Jacket getting hit with Flechette rounds rolls a total of 30 dice to soak (resist damage), does he roll 30 dice?
3: the cap happens after modifiers so anyone with higher than 20 dice can opt to take modifiers to reduce their dicepool to 20, wouldn't this promote a gun specialist to make called shots, to vital locations, and ignore armor? rather than just shoot from the hip?

I say it should only affect Attribute+Skill rolls. Soak rolls or Attribute only tests should not be capped IMHO. I don't know if the Missions Rules clarify this.

Using the 4 to 1 exchange rate decreases already the effectiveness of a dice pool. IMHO doing so should not be penalized further.

As I wrote above, I would only apply the modifier to skill tests.
Yes. The options for called shot however are exclusive. You can either avoid armor or aim for a vital spot.

your best bet would be to ask this over in the missions forums.

which thread? none appear to be for asking questions.

just start a new one.