Berea uses a second mandrel size for a few of their kits (8mm or "B") but only ba few kits use it and there are clones of those kits made by other companies that use standard "A" mandrel bushings . Look at the chart on the front page to find out which bushings are the same .
Actually, the "B" mandrel is just slightly larger than the "A"... enough to make the "A" mandrel bushings sloppy.... there is an 8mm mandrel... I have one, but haven't used it in years... think it's rusty now.
If you get your kits from most of the standard suppliers... CSUSA and PSI and their resellers, the standard 7mm mandrel is fine... Berea is the only supplier (and their resellers) that uses the "A" and "B" mandrels on some of their kits... usually, the catalog listing will tell you which mandrel you will need.
As mentioned, I now turn between centers, so my mandrels mostly languish in the drawer under the lathe.