Using an Atmega8 Chip with an Arduino Uno instead of an atmega328
1) yes the arduino uno can be used with a blanked at mega8 chip with the NG bootloader burned onto it.
You would set the board as NG in the IDE to use the arduino IDE (to upload sketches and so on) with it.

2) A crystal and capacitors is still required -the same values as used for the removed atmega328.

3) The smallest bootloader will leave 7.5kb of program memory left to use on the atmega8.

4) Yes