Can I use a technique before finding its journal entry?
Yes
Per the Wiki
It is not necessary to find the bookshelf explaining the Technique in order to use it: the player only needs to have an appropriate weapon and input the command.
Anecdotally, I have accidentally performed moves using a weapon. It will execute and I can go about using the new move and not needing to "discover" it in a journal.
The primarily limitation of the move is that they are typically tied to 2-3 specific weapons. You have to master it using those specific weapons before being able to use them across the entire class of weapons.