My approach to mama sans (I am defining this broadly to include any and all bar staff who try to assist you in selecting a boy) is, I suspect from most comments I read on boards, not the usual.
I view them as my source of local knowledge. They do know their boys. My objective is to leverage their knowledge to identify the boys most likely to be compatible.
While I have visited Bangkok regularly for over thirty years my visits have generally not been frequent enough to keep in touch with individual boys and the boys turn over faster than the mama sans, so I am more likely to meet the same mama san than the same boy. So I want to be, and generally am, on good terms with these ladies/guys.
In the bars which I regularly visit (X Size is one) I generally welcome the mama san, buy her/him a drink, ask her/his views on the boys, and give her/him a tip whether or not I off a boy. These ladies know that if my evening is a happy one, I will return the next night with a 'thank you' tip. If it is not so happy I may not return that trip or if I do, there will be no 'thank you' tip and the tip that night will be more modest. It is an approach that has worked for me. I have not had an incompatible boy for a long time. When I was winging it on my own 'infallible judgement' I had about a 30% failure rate.
As Vinapu notes the skinny ladyboy mama san in X Size is on a planet of her own in terms of lack of basic interpersonal skills . If the others are there she does not approach me. If by bad luck she is the only one on duty then I have to make it work. This I do by taking firm control of the conversation with a combination of the blocking hand signal and the polite but firm stare that suggests that if you annoy me the death of one thousand screams will be yours. She then answers questions and otherwise keeps quiet. She knows the boys, so her answers are worth having but, unlike when I am talking to the other mama sans I do not take her word completely (she is far too focused on that night, no thought for the future) and will leave for other pastures if my radar is detecting poor vibrations.
In bars that I do not visit regularly I will make contact with a mama san if they do not get to me first and again take control of the conversation (absent the heavy duty stare that is only required with the one particular mama san in X Size). If they are helpful they get a tip. If I am happy they get a 'thank you' tip the next night. And now I have another good source of information.
It has happened that a mama san has advised me, regretfully, that none of the available boys at her bar will suit me (I am arriving late) and has then taken me to another bar which her network tells her has boys I will like and has introduced me to the mama san there, for a happy result all around.