Welcome to the forum, Derek.
The longer a tile is, the more likely for lippage. It would be very helpful for you to grab the longest straightedge you can put your hands on and drag it around on the floor you want to tile. Twist it every which direction to get a feeling of how wavy your floor is. For tiles of this size, you’re looking to flatten out the floor to within 1/8” of flat over a distance of 10’. You can.....but don’t need to make the whole floor level...but you do need to make it as flat as you can.
Yes, you can intermix the two sized tiles on alternating rows. I’ve seen that before and looks decent. Mostly a preference thing. But if the tiles aren’t flat (many are slightly bowed up in the middle), the areas where you are forced to install the low ends of one tile near the high center of another tile will result in unwanted lippage. For tiles of this size, there’s an industry standard to not overlap adjoing rows of tile by more than 1/3rd the tile length to avoid this very problem. So, if you are going to intermix, the tiles need to all be flat.
