We used different flooring in our kitchen because the wood we have throughout the entry level of our home is oak and not a good match with our oak cabinets. We just used a matching transition piece at each of the doorways. It looks beautiful.

However, in regards to plank size, using a different plank size will look odd. We used the same plank size and it helped to keep continuity between the rooms. Ideally, your floors should all be installed in the same direction and the different size will break up the flow.

I don't know what the "rules" are, so I have no trouble following/breaking them....

When I moved into my house, most of the first floor (with the exception of the family room) had pre-finished oak floors in a medium tone which I didn't love. By and by, I replaced the breakfast area flooring with tile, and the kitchen with cork. When it came time to re-do the family room carpet, I wanted a darker, wider-plank wood floor (better suited the decor, shown below), so I put in new maple flooring. Replacing the oak flooring in the rest of the house was a non-starter, so we just made sure there were nice transitions between the flooring, width and direction notwithstanding. (A photo of 3 different flooring materials abutting is below.)

I like to think my flooring works -- it does for me. But, at least in my home, one factor is that it's most definitely not open plan. Each room has a distinct doorway and -- given my passion for color -- a different color. I'm not trying for a unified flow because it's not consistent with the design of the house or my delight in the different colors of the rainbow.

We have a 111 year old house that contains four different hardwoods. We have wide plank pine on the second and third floors, original hard pine in the den, oak in the kitchen and a higher-end original oak in the formal living/dining and foyer. One of our floor guys told us that it was common for Victorians to put the "fanciest" flooring in the fancy rooms and then the second-best in the less formal rooms and the cheapest in the places company wouldn't see, like the bedrooms. Not sure if this is accurate, but it makes sense in our house!

So, in a nutshell, IMHO it's totally fine and interesting to mix the floors up a bit.

I'm not a big fan of mixing wood or plank sizes. I like the idea of a common floor to tie all the rooms together. The great thing about wood floors is it is very easy to go to the store and buy a box of planks and lay them out on the floor. You'll get to see almost exactly what it will look like before it is installed. You can then decide if you like it or not, which is all that really matters.

My old house from 1923 has the original oak floors with walnut ribbon around the outside edges. There is an oak threshold and then the original edge grain pine in my kitchen. They run the same way, but are the pine is noticibly different in color and a little bit wider. Its all good.
The upstairs is also the original end grain pine not the oak.