Gambling halls and the like would be present, but characters populating them would be alts as opposed to someone's primary pc (reason being it's not the focus of the game and I don't want balls with 75% commoners because 'but I wanna go!')
Age-wise: see original post re:modern sensibilities for some things and that this is not going to be an ultra realistic take. No PCs under 18. I just don't do that, personally.
As for family sizes....they'll range. Some will be bigger some smaller. But I'm also going to steer away from houses of 1-2 because that leads to 'Well they have to marry in because I can't abandon my name' and people being endlessly unwed. Secondary reason being: I intend to allow same-sex marriage as long as it's not to the heir. More reason for larger families.
As for heads of house being NPCs: I considered it but it'd be hard enough with just a handful of families. No the way of encouraging drama right now (in drafts) is (tl;de version) 'scandal is encouraged but not forced however if you don't want scandal don't flaunt what you're doing else staff might visit it on you anyway'.
aka have affairs etc but don't expect to be able to waltz around in public with your lover and not get smacked in social standing for it.