I strongly disagree. Using Twitter bootstrap is OK. Copying other design is not OK.

Also, saying that design is a set of rules is like saying music or drawing is a set of rules. It's not. Design is a creative process. It's not about knowing how to use the right tools. Mastering Photoshop or Twitter Bootstrap doesn't mean you will successfully create a good looking design. I know how to use a lot of design tools, but when I try to create a design everything looks like garbage. If you're a coder, do yourself a good and hire a designer for your apps.