You can, but I'm not sure why you would, the buses go directly to the park entrances whereas the taxi would have to drop you at the parking lot and then you'd have to get a tram to the entrance. The taxis at the parks are not allowed at the entrance, this is just for the buses. To get a taxi after leaving the parks you'd have to go to the parking lot to get a taxi. They are also located at the TTC.
To get a taxi form the POR you'd have to ask bell services to call for you, they do come pretty quick. We've taken a taxi from our resort to another resort for dinner, just to save time. But really you'd do better to take the bus when going to the parks.