Pruning Ceanothus and Clematis Montana

Hi All  Two separate pruning questions please.

If I plant a ceanothus which is supposed to eventually grow bigger than I want it to be and therefore I prune it annually so it never fulfills its raison d'etre obviously I will stunt its growth but will it be damaged in any other way such as not producing its customary flush of blooms, particularly as the years go by?

I have had clematis montana before and know how rampant it can be.  Previously I forgot to prune it until it was about 6 years old and it did not like what I did to it.  I have planted a new one in a different spot and I think it sensible to prune it in this its second year after the blooms are finished.  The problem is that when I do this, because of the tangle of stems, inevitably I will cut some that I do not mean to.  Clearly I cannot trace every stem back to source so I know precisely what the damage will be.  So is it just the case that for the year from pruning to new growth I have to accept that it will look a bit ropey or is there some secret?

Really appreciate any advice.

Thanks on both.  Bob