Thanks for the helpful reply, Luděk!
I had created the 'ae' interfaces (ae0 - 7) beforehand, so that wasn't it.
However, I went to set the ae0 interface to 100m as you suggested, and Junos carped at me about the underlying interfaces not being set to the same speed, so when I set the phy intf's to 100m and then the ae0 intf to 100m, the candidate config committed, and the links came up! No need to do anything on the Cisco side. Traffic now passing and everything green & clean. Would think auto-config on the Junos side would negotiate 100m, but I guess not (because of the aggregation, I'm guessing - as I said, the int's when not in the agg linked and came up fine at 100m.)