Ive had a few prickly pears and I can attest to them being relatively mild flavour. What little they have bitter sweet and sour comes to mind. Id be cautious using this as the main fermentible leaving the drier bitter sour finish. You might be best using this more as an F-pac post or late fermentation as skarkman mentioned.
As for the fruit to use typical non grape or country wine is about 4-5lbs / gallon but with added sugars depending on ABv and body type your shooting for. With an ale yeast you could expect a higher FG some and some mild residual sweetess. Prickly pear would be one of those odd balls your stuck pioneering for the rest of us. You could drop a camden table let it sit a day or two and find the gravity of the straight juice then add simple sugars to up the gravity of your choice following an online chart.