1) There is a tactic for "Re-Roll 1 Hit Dice Result" and a tactic for "Re-Roll 1 Bullet, Alien, Jam Result"
After an attack roll, if one rolls a alien and wants to reroll that alien, only the Re-Roll 1 Bullet, Alien, Jam Result tactic can be used to reroll the alien?
Kilrah already answered this, but see the table on page 17 for where this is explicitly stated in the rules.
2) Hulk and his No Pain skill combined with the Elite Agent energy shield:
His No Pain ability states:
+1 SHIELD when defending against 3 or more HIT
Say 3 hits are rolled against an Elite hulk - does his No pain ability still apply, even though one of those hits is going to be absorbed by his energy shield?
I believe that No Pain activates in this instance. Section 7.3, p. 16 of the core rulebook states:
"If the defender has Energy Shielded Armor, he must first cancel hits by removing one point of Energy Shield for each HIT, before rolling the defense dice."
This suggests that Energy Shields are triggered as the very first step when defending, but No Pain triggers before that (or in other words, spending Energy Shields is part of what you do "when defending").
I thought Agent S published a more detailed list of combat steps at some point, but I can't find it at the moment.