Na sua pergunta, você nos deu dois casos de uso.
Primeiro, vamos analisar os termos críticos.
Flanqueando:
When making a melee attack, you get a +2 flanking bonus if your opponent is threatened by another enemy character or creature on its opposite border or opposite corner.
Creatures with a reach of 0 feet can't flank an opponent.
Enxames:
A swarm is a collection of Fine, Diminutive, or Tiny creatures that acts as a single creature. A swarm has the characteristics of its type, except as noted here. A swarm has a single pool of Hit Dice and hit points, a single initiative modifier, a single speed, and a single Armor Class. A swarm makes saving throws as a single creature. A single swarm occupies a square (if it is made up of nonflying creatures) or a cube (of flying creatures) 10 feet on a side, but its reach is 0 feet, like its component creatures. In order to attack, it moves into an opponent’s space, which provokes an attack of opportunity. A swarm can occupy the same space as a creature of any size, since it crawls all over its prey. A swarm can move through squares occupied by enemies and vice versa without impediment, although the swarm provokes an attack of opportunity if it does so. A swarm can move through cracks or holes large enough for its component creatures.
Swarm Attack: creatures with the swarm subtype don’t make standard melee attacks. Instead, they deal automatic damage to any creature whose space they occupy at the end of their move, with no attack roll needed. Swarm attacks are not subject to a miss chance for concealment or cover. A swarm’s stat block has “swarm” in the Melee entries, with no attack bonus given.
Ataque furtivo:
The rogue's attack deals extra damage anytime her target would be denied a Dexterity bonus to AC (whether the target actually has a Dexterity bonus or not), or when the rogue flanks her target. This extra damage is 1d6 at 1st level, and increases by 1d6 every two rogue levels thereafter. Should the rogue score a critical hit with a sneak attack, this extra damage is not multiplied. Ranged attacks can count as sneak attacks only if the target is within 30 feet.
Então, com aqueles claramente definidos, vamos ver os casos de uso;
Pele do Enxame:
Esse poder afirma claramente que você não pode usar "nenhuma de suas habilidades ou realizar outras ações além de controlar os enxames", de modo que isso excluirá ataques furtivos completamente.
Transformação Verminosa:
Este poder não limita suas ações ou suas habilidades. Portanto, qualquer ataque que você fizer poderá se beneficiar de um ataque furtivo, desde que você atenda aos critérios. A ação padrão que ele fornece também pode se beneficiar do ataque furtivo.