You should be fine taking the bus.

We do a couple early, pre-park ADR's every trip, and busses are always running.

Although they are not as frequent, there is pre-park transportation to each park. I don't know why the Disney site sticks to the "one hour before park opening", since I think many of us have seen MANY busses far earlier than that. I know it hasn't been my experience at least. We've bussed to MK for 8:05 breakfasts many times without any issues. Since they aren't as frequent, you do need to be at the stop earlier than you think, since you don't want to miss one.

For 7am, that's going to be tight, but anything around 8am or later, there will be MK busses running. I think the earliest I've seen them at POR/POFQ is just before 7am. Quite often, the driver will take people to their ADR's at 'Ohana and Chef Mickey's after dropping off at MK, but this isn't guaranteed.

Taking a cab isn't expensive, and it is quicker and easier, especially early in the morning, but the busses are an option as well.