Pick one. Go with it. Whichever you pick is the best, if they fill the same function. Regarding how strict you have to be:
I chose some of my persons to be extremely off-system. For example: 58: 5 looks kind of like a lightning, and 8 = H, so Harry Potter. 35: 34 kind of looks like BA and Belgariad is one of my favourite books, so 34 is the main character from that book, and 35 is another character. So far I see no more difficulties remembering these kinds of links than the “perfect” 23 = Bill Clinton links. In fact, Dominic O’Brein even writes in the book I have that you should first assign the numbers that you can directly relate. I get the feeling that the character conversion system is just to be sure to have a way to fill the gaps.
Some advices that I haven’t seen mentioned before:
Just write down a list of the names at first, don’t memorize it. That way it is very easy to change your mind. My rule is: Once I have used it in an actual memorization, I’ll stick to it. (Don’t worry about the reason that you’re effectively making a decision that will stick for the rest of your life. That forever until you die you will associate this number to that person. Forever.)
Choose characters that you really know. The more you have interacted with the characters before, the stronger sense you will get from them, and the easier it is to connect them to whatever weak link you choose. I much rather have a weak link with a strong character than the other way around.
Because of this reason I ended up changing some of my numbers. For example 17, AG: Art Garfunkel: I have only seen photos of him, and his face didn’t really stick in my mind. So instead I changed 17 to Patrick Jane of The Mentalist. 17 is “the most random number” that people will just say if you ask them to give you a random number. It’s the typical kind of mind trick that he would use, although I haven’t seen him use that one specifically yet.
When I came to the point that there were maybe 40-30 names left, I started going from the other side instead: I wrote down a list of names that I would really love to have in there, and then found a way to get them in there.
Maybe not the best for remembering, but: choose people you love to have in your head. I have avoided people that I hate as much as possible, and really tried to fill my list with lovely characters and role models. You’re probably going to spend a lot of time with these imaginary friends dancing around in your head, so might aswell make their stay pleasant for you. My list includes people like Dalai Lama, Gandhi, and just random lovable characters of different shows and books.
Try out the action/object on your most difficult characters: I have pikatchu and King of Hearts in my list, if I can imagine THEM doing the action, it is distinctive enough.
Tl;dr: Don’t worry about the rules, just make any kind of connection that you can find between a number and your favourite characters. The rules are there as a crutch if you have problems doing this. You don’t have to show your list to anyone, so the links can be as stupid as you want them to be, and break any rules you want to, as long as they’re memorable. It really doesn’t matter.