Wouldn't it be best to first see if the crystal malt can actually convert itself? You can test different lengths of time steeping just crystal malt; maybe after 20, 40, and 60 minutes, and even longer if you have to. This may tell you whether crystal can actually convert itself, and, if so, how long it would take.
I am assuming the length of time would matter a lot because specialty malt doesn't have much diastatic power, and the added time would allow the enzymes to "get around" to all the starch. (Unless enzymes denature over time?)
It seems many extract brewers do not wait all 60 minutes while steeping. I know I was instructed by my LHBS to just wait 30 minutes. Doing one test at 60 minutes may give you a false impression because at that point there may have been enough time for the conversion.
Wouldn't the test I described answer directly whether “crystal malts require pale malt for adequate extraction"? If it can't convert itself, then you would want to move on to step two to determine whether base malts would help get adequate conversion, and how much they help.