No, I'm pretty sure there aren't any pre-requisites for getting Boldin. Having a higher Charisma level only affects the amount of gold you can receive for selling items at shops, and makes items cheaper to buy in shops. But this skill is fairly useless, as even with an almost full Charisma level, the difference is redundant. You save about 100 gold when buying stuff, so it's just a crappy attribute altogether.
What other characters do you have in your party? If you have others, you can try selecting each one and then try to have Boldin join the party. Also make sure the party isn't full, of course
The only other reason that a character would refuse to join you, as I've found out in my many travels, is if you let them leave the party too many times - maybe two or three, it could be different for different chars. But this probably wasn't the reason for your dilemma. And just to test it out, I fired up a game of AS, used makechar to roll my hero as low a Charisma as I could get (6 in this case), and then went straight to the castle and picked up Boldin. It worked fine. So idk what's the problem for you, try the stuff I mentioned up there.
And btw, Gelinda's castle is quite tough for your characters at the levels you mentioned, they definitely wouldn't make it. I suggest going to Crystal and completing quests there first like Lord Chancellor Drebin's quest and Orlando's quest. Hope this helps!