Yep, talk with fiefless clans and 20 relationship is enough to be friends and get the dialog option. Recruiting fiefless clans when clan leader only has 20K money or so is pretty easy.
What I usually do: I start building relationship with clans I want to recruit in the future. Then I join a neighbor kingdom and help it to conquer the fiefs I want for myself, and ensure that I am getting all fiefs from my “friendly clans”. After some time, I become king keeping all my settlements, and I talk with the clan leaders and recruit them without problems and without having to pay much/any money.