I honestly didn't have any problem sliding the PexAlPex over the O rings.. I used a 'rounding tool' to make sure the tube was round, and put a very slight bevel on the inside of the Pex.. with a pocket knife,, not much of a bevel, at the most a 45* to the aluminum...
I didn't have to force any fittings,, they slide on with a little spit or water.. you could use silicone or similar lube.. use a fitting without O rings as a trial tester,, just to make sure your pex IS round,, this would be the only reason for needing to force the fitting..
If all else fails.. get a bigger hammer.. works with dentistry,, but not plumbing !!