Quantum Singularity
A quantum singularity is indeed a tiny black hole. Although tiny doesn't really do it justice, subatomic is more accurate. There are also theories that similarly tiny wormholes could exist.
I suspect that the Trek writer's thinking is something like this:
The bioships create a quantum singularity (QS), which should be fairly easy, as it shouldn't be too long before we're making them in particle accelerators. This singularity is used to create a wormhole linking fluidic space and our own dimension.
Normally a QS "evaporates" almost instantaneously, so the bioships must sustain and then enlarge it (large amounts of negative energy would be required). The wormhole would be enlarged to allow ships to pass through, then, it would collapse once the negative energy is turned off.
8472 target aggressive ships
What is this based on? 'Scorpion' implied that they target the weak: "the weak shall perish". And, from my own recollection, the episode with Chakotey, showed us that species 8472 thought the federation was aligned with the borg. Once Janeway proved otherwise (by giving nanite tech), they no longer wanted to fight the federation.
The impression I got was this:
Janeway was completely in error regarding species 8472. She jumped to the conclusion that 8472 was a greater threat than the borg. She based this on a few limited encounters in which she failed to establish contact. Kes got the thoughts "the weak shall perish" and "its an invasion"... from a few (one?) alien pilot. Janeway stereotyped the entire race based on this limited encounter. Species 8472 was currently engaged in a war against the borg, and the aliens she encoutered were soldiers in a battle... the thought that these soldier's thoughts might be a little overly agressive didn't occur to Janeway.
The episode with Chakotey showed the species 8472 was not planning on destroying the entire galaxy, only destroying the borg and its allies. Had Janeway followed first contact procedures, and persisted in attempts to communicate regardless of a few culture misunderstandings (as she did with the Chakotey episode) she might have found a valuable allie to destroy the borg once and for all.