It is possible to connect a 6-ohm speaker to an 8-ohm amp. While the internal impedance of the speaker is 6-ohms, the output impedance of an amplifier is essentially zero. This is due to the active control circuitry in modern amplifiers that makes impedance matching in electronics unnecessary.
Impedance is a measure of a circuit's resistance to electrical current. The impedance rating of an amplifier measures its internal resistance, not the output resistance. An amplifier's active circuitry controls the amp's output voltage, ensuring the correct amount of power is delivered. Loudspeakers with a lower impedance draw more current and generate more heat than speakers with a higher impedance. As long as the impedance isn't below 4-ohms, the amplifier drives the speaker without having too much current flowing through its output transistors, which could cause them to overheat and shut down.