No, hamster shows can be for all hamsters, they just have to conform to the 'standards' for that colour and breed. In the pet section hamsters are judged on condition and tameness only, rather than colours etc. In the main show, the hamsters are awarded points based on
colour and markings (30 points)
type (25 points) - the body shape and head shape which should be broad and cobby, with a head well set into the body with a broad skull and short face.
fur (20 points) based on length and density of the fur including the belly fur
size (10 points) the hamster should be as large as possible without being fat
condition (10 points) the hamster should be fit, and healthy
eyes and ears (5 points) large eyes, widely set apart. Large ears also set well apart and carried erect when the hamster is awake.
entering in the main show does make for a longer day as the hamster needs to be in the show first thing and then stay until all judging is over, but its a great way of meeting people and finding out more about hamster showing and 'what makes a good show hamster'. I've seen people who enter their animal in the pets firstly and then at a further show enter in the main show on the advice of the pet judge.
The advantage of the pet section for a 'newbie' is that you can just arrive before the pet section is judged (usually in the afternoon).
Imogen is a cute girl and I would encourage you to take her along either way.
PS - we love people who give homes to rescues - well done you!