There are two different types of T5 bulbs available. You would want to use one or two T5NO bulbs for low light.
T5HO would be too much light for a low light tank using a canopy since you wouldn't be able to raise the bulbs high enough above the substrate to sufficiently reduce the intensity.
The problem with T5NO is that selection is limited. Coralife may be the only manufacturer that sells T5NO fixtures and bulbs.
In general I don't think it is a good idea to enclose any fixture in a canopy. For a canopy get a retrofit kit install it in the canopy and add a fan or fans if needed.
There is no reason that you could not use T8 bulbs in the canopy to achieve your goal. Four bulbs would probably work well for you if you can fit them in the canopy. The chart you linked to doesn't cover every possible scenario. Pretty sure Hoppy only included bulb/tank height combos that he tested with a PAR meter.