The only need for a prerinse may be to prevent airbells. If you get airbells without using a prerinse, then give a short (one minute or so) prerinse. A prerinse for that reason can be used with Xtol if need be.(posted 8516 days ago)Quite a few people have had trouble with Xtol, particularly at high dilutions, thus Kodak's recommendation of 100ml stock. While you could use less, you'd be getting into the area of a known problem.
As for D-76, I've found no difference in EI, CI or curve shape between using 100ml stock and 50ml stock, as you'd probably want to for the 1:3 dilution. That's with HP5+, Delta 100, TMX and Fuji Acros 100.
BTW, the volume required per Jobo is that amount of solution needed to cover the film; you can use more, although the CPE-2 is limited to 600ml.