Personally i dont think there would be much romantic sentiment attached. We all know mycroft is a queen. I see him as someone who likes to sleep around. Like a bunny lol but not much emotional attachment. (That being said i do love him paired off with anthea buuut thats a whole other ball game.) i can see him having multiple partners (except john/sherlock/lestrade….not against mycroft/lestrade by the way, i just don’t understand it since they dont share screen time except maybe once or twice.)

He is not alarmed by sex & teases sherlock about it & from what sherlock said at the wedding “ive either caught you in a compromising position or exercising” i would think sherlock is used to walking in on mycroft with a random partner on more than one occasion.

I also think both mycroft & sherlock rely more on bdsm than emotion. Irene adler has sherlock in a tizzy & benedict himself once said that sherlock is no longer virgin once he meets irene & if we go by all the books, i dont need to mention nero wolfe, their son. Mycroft thoroughly enjoys watching sherlock get beat up & sherlock doesnt seem to mind so much either.

So anything could happen with smallwood but at the same time, he does owe her so a drink is fine. He is better at having friends than sherlock is. I could see him having a fling with her but unless he is getting older & feeling a need to settle, i dont see it lasting much beyond anything quick & physical.

As for their age difference, it doesnt matter so long as both are adults. They are both well over the age of 21 so it should be fine.