Hey, can you use the same kind of screen printing machine for both t-shirts and posters?
Sort of. To do paper printing properly you need a vacuum table. But you can use a t-shirt carousel, with some difficulty. It's not ideal, but it can be done.
Also, can i cover an entire shirt with a design using a hopkins 6 color 4 station press
Not as the machine comes standard. I don't recall if Hopkins is side or rear clamping, but if you can get it rear clamping... then grab new platens and a set of oversized screens and you'll be able to do oversized (allover) printing.
Both of these applications are reasonably advanced if you're new to screenprinting (one of those "if you need to ask the question, you may not be quite ready to tackle it" situations). It's not that they're especially difficult, but they'll be prone to excessive errors, and if you don't have the rest of your process down, it will be impossible to separate specific problems from general ones.