Firstly, there are countless other female Borg drones, watch "Unimatrix Zero"
and "Survival Instinct" as 2 examples.
as for your main question the short answer is NO (but i may be wrong).
This is why I believe this:
The queen says 7 was planted on voyager in the 'Dark Frontier: Part 2'. This could have been a lie but I don't think it was (when the queen did lie in that episode, 7 knew instantly. The Borg are bad lairs due to inexperience with the concept).
The 3 cubes voyager approached with terms for negotiation, 1 happened to have a human aboard: pure coincidence, (This is when I think that the plan was conceived, to put a Human/Borg spy onto voyager, they saw an opportunity here). A human who could represent the Borg (like Locutus). They knew Janeway would not be easy to negotiate with so they beamed her to the cube with the human onboard (and only a short walk away).
7 was ONLY activated when Janeway refused to obey the Borgs methods -
Borg: "human communication is inefficient, you will be assimilated",
Janeway: "we must do nothing choose a representative or the deals
off". If 7 was a queen she would have been activated as a
representative from the start.
7 does have a unique physiology for a Borg (the breasts mainly), but
we don't know when she came out of the maturation chamber (yesterday,
a year ago, who knows), the Borg seem to grow with the implants,
which is shown in 'TNG:Q who', she might still be maturing in the
alcove: hence all the pipe that disconnect when we first see her.
As far as we know the queen is mainly a brain and spinal column, from
the 2 example we have, the movie 'First Contact' & Voy:'Dark
frontier'. Both of these queens were different people (in and out of
universe). This strongly imply's shes a processing node for the Borg
and just an avatar when needed. If this is true 7 would have been
activated the moment Janeway was beamed aboard.
Non cannon but queens are only picked from 1 species, one where the
women have great minds (Can't remember which novel, sorry). This isn't cannon so it can be ignored if you like.
One last thing and this is a big one, a queen listens to and gives
instructions to the collective, whereas 7 in Voy:'Scorpion Part 2'
gets told to take control of the earth ship, to which she replies
"understood". This isn't how a queen behaves at all from what we've
In short, none of this suggests a possible queen, actually the exact opposite, she just happened to be human plus shes on one of the cubes, and the Borg saw the opportunity to prey on those pesky human weaknesses (emotions). They knew they couldn't assimilate by force (the threat of losing the 8472 nano-probe data), so they used what resources they had close by, a human/borg to prey on the 'enemy weakness', (that seems very Borg to me, but she was simply a drone until they realized this).
This is my understanding. the Borg simply saw an opportunity with a human/borg close by and used it. Hope this helps and like I said it could be wrong, this is just my understanding of the events.