Using older lens with adapter on micro 4/3 camera?

I'm planning on purchasing an Olympus micro 4/3 soon. I also plan on purchasing an adapter to fit my older nikon AI lenses for using with the system.

1. I've seen several types of adapter varying in size, some seem larger and thicker, keeping the lens actualy further away from the sensor. Does the thickness or added length change the effective focal lenght? For instance if I use a 50mm or 28mm lens with an adapter that's around half an inch thick?

2. Will I have to use full manual mode? Will the camera's onboard metering allow for me to use it in A or S mode?

Added (1). Also, I know that panasonic 4/3 lenses are compatible with olympus but how about sony nex lenses?

1) The effective focal length will change (multiply by 2), but this change is due to SENSOR SIZE. While technically the focal length changes with the distance of the lens to the sensor plane, this actually is irrelevant as if a lens is not correctly mounted at the right distance it will not focus on anything.

2) Since there's no mechanical connection between camera and lens. Only M and A (aperture priority) modes will be available with stop-down metering only. Only Pentax has legacy support for lenses in all modes.

3) NEX lenses are incompatile for two reasons. Firstly these lenses have a shorter flange focal distance than the MFT mount, hence you will not focus to infinity. Second is that it's a modern lens with electrical connections only and therefore you will not have any ability to control the aperture (and therefore no control over exposure).

Also you should note that any Olympus made FourThirds and MicroFourThirds lens is also compatible. As are the Leica M and R mounts and the Olympus OM, and Canon FD mounts.