In regards to the reason for Hal choosing Irma, they were quite shallow but in no way sexually motivated.
Irma was not the only seventh-rank Alchemist that Arya knew of in the Silva Duchy and while she had shown to be far more dedicated than the rest of her fellows, she was not the indisputable best although she did quite well for herself.
Anyway, if anything, Hal had chosen her at random...
His most important quality in who he recruited for his pill business was that the fellow was female as he was much more comfortable with allowing female access to the Harem space as opposed to a male and even then, there was a good number of female Alchemists that he could have chosen from.
"You are really taking advantage of my calm and level head. You are threading very close to the point where I stop restraining myself and eject you from my home. Your mysterious means be damned.!" as Irma spoke, the storm about her head got stronger.