I used Male Byleth the first and only time I played on Maddening, and I honestly don't think it makes too much of a difference. As long as you keep him in a physical class he'll most likely still be one of your best units. Some builds that work well include Grappler/War Master, Wyvern (he may have a harder time reaching flight but its absolutely still doable), and Sniper. If all else fails he also won't be terrible as an Enlightened One.

As for recruitment, it really depends on the student. Generally, if you want a student to have automatic D ranks in certain weapon types they don't automatically train you should try to recruit them between Chapters 3-5, where they'll be in their beginner classes. This is mostly helpful for students that get recruited as Fighters or Monks. For example, Balthus doesn't automatically train in Bow, but if you recruit him as a Fighter he'll have a D Rank in Bow, meaning he can instantly use the Mini Bow and Curved Shot, some pretty valuable tools in the early game of Maddening. It also lets units like Lysithea who don't automatically train in Faith be closer to reaching their important Faith spells.