Forex is traded over the counter, not on an exchange. Therefore you must do your due diligence when selecting a broker, or you risk being front-run and ripped off on the spreads.
You need to have a margin account to short stock. Technically you're always using margin when you sell a stock short, but you can limit your position size to less than or equal to the cash value of your account if you want to avoid being leveraged.
Most retail brokers charge ~10% annual...