Is the heat tape all going to be the same size? Because then it would be able to regulate them the easiest. Different heat tape sizes can still be different temps even when using the same power strip for them.

The biggest problem is going to be that even if the heat tape is all the same size, the difference in thickness on the tubs may affect the belly temps on them as well...
For example, on the shoebox, the plastic won't be as thick as the 56qt. So if you regulate it to 90 on the big tub, that may be making the temps on the smaller size tub come in much higher. So as Dr Del said, dimmers may be needed to keep those temps coming in right.
And then there's the problem that comes with ambient temps and such as well. You'll need to monitor them all pretty closely to make sure the different shelves are maintaining the proper temps.