Query: So then how do you get HK-51 for those low level characters?
Statement: I was under the impression that if you're subscribed, you get HK-51 for all new characters. I've seen some say that HK makes leveling easy. So they have him while leveling.
As said above you select character perks in your legacy panel and there you can unlock him, there will be 2 boxes, one is for unlocking him on a republic character and the other for a imperial character.
You need to be on the character you want to unlock him on and it will cost 1 million credits OR 350 cartel coins.
Sadly there isn't a option to unlock him on all character, you have to unlock him on each one paying full price on each character.