A little bit of experimentation isn't a bad thing. It'll help you a little later down the line when you need to take a project from the middle phases of development, all the way to the final product.
If you want 2 handy pieces of advice for using zbrush:
1: Take a lot of time to nail down the form development of each character. Often I like to a quick massing of the character, then go up to several levels of detail so clay brushes behave naturally. I nail down all the major forms with clay brushes and sharp line brushes, and then go back down to just a few subdivisions to move, inflate and orient each feature properly. From there you slowly move upwards. Going to super high levels of detail and trying to manage everything from that level, is really tough, and generally leads to bumpy meshes. To nail in sharp mechanical features, using high levels of detail, and masking is a good way to block in those forms.
2: Invest a bit of time learning the topology feature in zbrush, and the projection feature to keep your meshes cleaner. Often you'll find with certain meshes that the geometry doesn't flow in ideal patterns across a surface, or you might be short on detail entirely. Creating a mesh using custom topology and transferring your work over is a great way to get things cleaned up, and you'll be less reliant on using others base meshes if you want to add additional features.
It's demonstrated nicely in here:
As others have mentioned, be wary about how you spend your money. This is sort of an emerging industry and there really aren't any set standards for curriculum. It's not saying that there aren't good schools out there, but there are a heck of a lot of programs and courses that will leave you with next to nothing when you graduate. Really research what you're getting into and probably plan around investing in some books, DVD tutorials, and even budget some time where you can do independent study.
Your 2d work is great and it's apparent that you have quite a bit of artistic talent. You just need some time and effort to develop your skills, but you'll get there if you just stick with it. Best of luck with your upcoming deployment!