Yes, depending on the mix ratio, you will end up with an eggshell or satin finish.
I've done it without any ill effects. It just dulls the semi-gloss a little.
Either oil or latex primer can be used over interior latex paint.
It's not recommended to mix latex satin exterior paint with latex matte flat interior paint. The two paints have different formulations and finishes, which can lead to adhesion issues, uneven appearance, and varying durability. If you need a specific finish or color, it's best to choose a compatible type of paint designed for the same environment and application. Always consult the manufacturer's guidelines for optimal results.
You can make it flatter, but not totally flat by adding a bit of talcum powder. Very common technique in modelmaking.
The cast of Interior Latex - 1999 includes: Frances Hearn as Suzanne
Possibly the most common use of latex is painting interior walls.
You can use interior Latex pearl paint on your bathroom wall. Pearl paint adds a special finish to the wall.
Acrylic and Latex
Bob Vila's website rated interior paints as okay, better, and best. He rated flat as best and satin as better. Flat was especially recommended for ceilings. For trim work, satin was rated better and flat was unrated. http://www.bobvila.com/HowTo_Library/Selecting_the_Right_Paint_Sheen-Painting-A1951.html
Latex-based paint for interior walls has several advantages, including quick drying time, easy cleanup with water, low odor, and durability.
Usually it can be, especially if both come from the same manufacturer.
You cannot polish fully cured interior acrylic latex paint, it does not set up well enough to be polished. It is not designed for this and doing so can ruin your finish.