What chapter are you in? At a certain point in the story, Rex obtains a healer blade that would make Floren somewhat redundant. As for 4, you generally don't want to mix healers and tanks; Having healing blades reduces the amount of aggro generation and clashes with the tank's mindset. Overall, I'd either leave Floren as is or move him to Nia.
I'm in the post game (85+ hours in). I'm dont want to take on healing responsibilities as Rex - like you say, he gets a great healer, but I'd rather leave healing to the AI.
Mythra is too good to constantly swap out for the healer I think.
No.4 I have set up with her two story blades and Perceval, so I'm all set there.
I've got Nia outfitted with Dromarch, Boreas and... bizarrely, Dagas. I've seen some people say having an attacker on her has worked well for them, so I haven't really thought about it until now when I got Floren.
I need to sort out Rex to be honest, I just don't like Roc that much to be honest. I don't hate him, but yeah, I'd like to leave him behind.