If I get 33% pierce from Razortail and 77% from Pierce skill, will all my attacks always pierce?
If I have 60% critical strike from skill and another 40% from gear will all my attacks always crit?
I know crushing blow works at a fraction of the mellee value but can I both have a crushing blow and crit at the same time or do they overwrite one another?

Yes, piercing % from different sources are added additively. So that is 100%, or 95% since that is the max chance to hit relative to attack rating.

Critical strike and deadly strike are added dividedly. 60% from one source means there's 40% left. 40% out of 40% is 16%, tha means 60 + 16 = 76%.

They do not overwrite, can be used together. Ranged crushing blow works only at 12.5% on normal monsters and 6.25% on bosses, unique monsters and champions. So it is not very effectful, but it does help if you can add some without sacrificing much damage.