Interesting comments, I agree with many. By way of emphasis, I'll add my own thoughts.
The species you choose to create bonsai can dictate the size of the tree for horticultural reasons. For example, Acer rubrum, the north American Red Maple has fairly heavy twigs, and seems to always have constant long leaf petiole that does not reduce even when you are successful in reducing leaf size. They work well for bonsai over 24 inches in height, but look odd in leaf when smaller sizes are attempted. Same for sugar maple, A. sacharrum. If you try to do small bonsai with these 2 species, in the end these will only be "show trees" when leafless. So horticultural concerns can drive design and style choices. Some species are boring left to their own normal growth patterns, but do become interesting when technique is applied.
Some trees are not easy to style in a style that the mature form of the tree in the wild. For example, Pinus strobus, Amer. white pine, near me there are many glorious, tall, old,
stately trees with very horizontal branches and straight trunks, perfect models of the formal upright pine. Yet trying to create one of these in miniature from P. strobus seedlings or nursery stock will drive the average person to madness. However, a JWP, P. parviflora, is a perfectly good stand in to create this type of image. Similarly, live oaks in nature are magnificent gnarly old trees, but very difficult to tame to create a replica from a live oak seedling. Yet a boxwood can nicely duplicate many of the features of a live oak in nature, and it lends itself well to smaller size bonsai.
So horticultural concerns do drive many of the size and design choices. Some species such as juniper and boxwood are the modelling clay of bonsai, they can be sculpted to just about any shape and form. They can mimic a wide array of shapes seen in nature, they can even be styled to resemble their natural forms in the wild.
I would first try many different species of tree, and see what grows well for you. Then from these choose what you want to use as your "modelling clay". As to styles, naturalistic, or stylized, is up to you.
To echo what was already said. While there are "no rules", if you already have a good grasp of artistic design and display, if you are not sure of what to do, The Japanese rules for bonsai design and display are perfect, and well worth learning and referring to when you are uncertain of what you need to do to improve your own trees. Chinese design is distinctly different than Japanese, but it too is "perfect" when taken as a whole.
So bonsai can be styled any way you want, but the effort must be a collaboration between the artist and the tree, the tree's horticultural habits will limit what you can do.