Busboy and busgirl are both correct, but busboy is a more common and older term. There isn't any rule about separating the words of job titles for female positions.

I've never heard of a bus girl. A busboy is a job in a restaurant. I worked in a restaurant as a busboy. There were no bus girls. At any rate, the term 'busser' is the usual term in the U.S. I'm guessing busser is used for both F and M. It sounds neutral to me. I suppose there are bus girls but I've never seen any. At any rate, busgirl, bus girl, busboy, bus boy - it makes no difference how you choose to spell it. Salesman, sales man, saleslady, sales lady....

No. Well it's shorter, that is clear. But quite why you think it is more respectful, I don't know. It's 'text speak.' Not standard English. It originated when you had to pay for every letter of a text message you sent.

thx is a more respectful and shorter way to say "Thanks". Is that right?

feminine-relating... probably "female related words" would be better thx...?