Desert Tropicals Dot Com
One thing I have noticed is that their hardiness ratings can be very conservative. For instance, they rate Aloe maculata (A. saponaria or soap aloe) as a zone 9b plant. It is one of the most hardy Aloe species here in Houston area, easily tolerating year round rain, not-so-perfect drainage, and occassional temperatures to 15 F in the northern areas.
In a way it makes sense to be conservative because plants vary over their range, and perhaps some isolates of A. maculata are hardy only to about 25 F, whereas others might be able to take 10 degress F more cold.
The way I've used Desert Tropicals to advantage is to mentally subtract at least 5 degrees F from their cold hardiness ratings if I feel like pushing a plant. So, a zone 9b plant become 9a in my mind. Only time will tell if my strategy works.
So much of cold hardiness depends upon duration of cold, wetness, soil drainage, how many nights of cold, how well acclimated a plant is, etc. So, I'm sure I'll have some failures and hope I'll have some happy surprises.
No doubt, very few Aloe species have evolved to deal with prolonged temperatures below 25-30 F.
Here is a link that might be useful: Desert Tropicals