Mix till your heart is content. It is your house and you can do whatever you like! :-) Usually 3 different wood tones in a room is enough. More than that and it begins to look confusing. Would love to see pictures.

I think you definitely can have different metals in your house, as long as the hardware is in a consistent style. For example, if your kitchen is rustic/country, then oil-rubbed bronze or a distressed brass or pewter could work. If your room is more formal, then antique brass or polished nickel may look better. I have old brass hardware on all the doors in my 1920s Colonial Revival home. The rooms have some pewter and polished brass on the cabinet drawers. However, in the master bath I have white cabinets and marble tile, so I chose polished nickel. In the kids bath there's chrome, and in the kitchen white cabinets and dark (almost black) ORB hardware. It all works fine and adds some interest to the rooms so they aren't all matchy-matchy.

That depends is it a closed off room will the new trim butt up to the old . It's best to have all the trim match

Absolutely feel free to switch it up. It's totally separate from the upper floor and an opportunity to make it interesting. Personally, I think it would be a bit on the boring side to have everything match.