I am posting the same reply as to a similar question about bushings.
Unfortunately, there are variations in dimensions for similar kits from different sources. Sometimes it is only a couple 1/1000ths of an inch, but that can make a difference a good fit and a sloppy fit on the mandrel.
My advice for those starting out is to always buy everything from the same supplier, and stay away from the general retailers who may not be getting all of their kits and parts from the same source. If nothing else, you will know who to go to if there is a problem. Save the mixing and matching of parts until after you have made a few pens and accumulated a few additional tools, like the dial caliper that Ed suggests.
In otherwords, if you bought your kits from Wooodturningz, then get the mandrels and bushing from them as well. That way you will know everything will fit. A beginner can have enough problems with turning, sanding and finishing without adding things that don't fit to them.