[HPforGrownups] Animagi: can they chose what they will become?/How did Rita Skeeters become an Animaga?

[Yoris]

No: HPFGUIDX 35500

"Does the animal one turns into as an Animagi reflect your personality? Very
well deduced, Narri! I personally would like to think that I would transform
into an otter, which is my favorite animal. Imagine how horrible it would be
if I turned out to be a cockroach!"

This is a part of a JKR interview and it suggests you can't chose what you'l
become, but Peter Pettigrew did become a rat so he could push the 'button' on
the willow, so how did they know one of them was going to be a small animal
capable of pushing the button without getting beaten? Can't they chose what
they become but will they know in advance? if they can't chose what they
will become, how did Rita know she would become an insect? (because any big
animal she could turn in would be completely useless)

Further more this brings me to the question: How did Rita Skeeter became an
Animaga?
 I mean it took 3 of the most brilliant students hogwarts ever had (Remus,
James and Sirius) working together for 3 years to become animagi... even
Peter Petigrew, although described as the least of the 4, still a powerfull
wizard (he kills cedric using AK, a spell that needs alot of power behind
it, moody says if all children in the classroom would do it simoultaniously
still he woudn't be harmed) could only manage to do it with ALOT of help
from his 3 friends. So how did Rita Skeeter, in my opinion a witch with
below average magic capability;s manage to become an animaga without help?
(she probably hadn't (m)any friends at school since she always was
gossipping about everyone and she couldn't ask the MOM cause she became
unregistered animaga)