These skills like Swarm are generally in a separate Proficient / Skilfull section or whatever they are called, and I can add the same there. They stack up. Sometimes certain items have, for example, +1 Arc Legion and +2 Arc Legion at the same time. I wonder if then you can add +3 Arc Legion with augment?
See, this is what I'm unsure of. In 2038, different grades of the same affix can't be stacked on one another. So you couldn't get two "Able" affixes of the same skill, or a combination of "Skillful", "Proficient", etc of the same skill. In +crit's case, you shouldn't be able to stack Namesis with Avenger's, since they're the same affix in different grades. But since you can, I'm unsure of what the exact rules are in Steam.
On sections, just to be clear. If Steam follows the same rules as the original and 2038 do, then you *can* stack inherent and Unique affixes with augments. For example, consider this item:
https://wiki.london2038.com/Remnants_of_GrregasInspecting it in-game reveals that the +skills it has are unique. That is, they're not listed (as "Able:"), so they're parts of a unique affix set. In such cases, you can augment "Able" affixes on top of those with no issue.
This might be what we're seeing here, but I'd need screenshots of the items' inspection windows to know.
Only in this whole table of affixes which is mythic and which is it rare? Nemesis is mythic a Aveneger rare?
This should help with that:
https://hellgateaus.cyou/wiki/AffixesThough, again, I'm not sure of Steam's stacking rules. I only assume the rule I have in mind doesn't apply here, going by what you said - and the screenshot above, where a Glyph has both Nemesis and Avenger's.