Molly questions

In my tank, there are two sailfin mollies, two black mollies and one
silver molly. There are also three fry of varying sizes, all black,
with the largest having some pale-coloured patches on its abdomen.

I know one of the fry came home from the LFS, but I don't know where
the other two came from. I've been assuming that one of the mollies
in the tank arrived pregnant, but most websites seem to say that a
pregnant molly is very *visibly* pregnant - i.e. it's not something
you'd miss - and I've not noticed any of mine looking pregnant. Is
pregnancy in a molly always unmissable? And could fry born at the
same time in the same tank grow at different rates so that they are
now quite different in size?

(It just occurred to me that maybe the largest is a male? - I'll have
to wait for it to come out and see if I can tell.)

One of the black mollies is male, one female. The male has a much
larger dorsal fin than the female. Is this a characteristic of
gender, or is it something he's acquired through breeding? When he
flares it, his dorsal fin looks almost as impressive as the male
sailfin's dorsal fin.

Behaviour - today I've noticed that the male black molly is being
very aggressive with the female black molly - prodding her to the
extent that he's actually knocked her over sometimes. He's pushing at
her just behind her anal fin. She's taken to hiding down near the
bottom of the back of the tank, which made hubby suggest that maybe
she was giving birth, but I wonder if she's just trying to get some
peace from the male LOL. She looks plumpish, but certainly doesn't
have the "almost square from the front" appearance I've seen
described on websites.

If we do get more fry (and from today's reading I think it's almost
inevitable, and yes, I can see a larger tank in my future LOL), I
know we should use a breeding trap of some kind. What kind is best? -
the nets which allow the water to flow through? - the solid perspex
kind? - the specific breeding ones that let the fry fall through into
a separate compartment from the mother? And when should I move the
mother-to-be? I know it should be as late in the pregnancy as
possible, but is there any particular kind of behaviour I should look
for?

I am really liking this fish-keeping business by the way :-)

--
FishNoob