I am assuming your are going to get the enclosure for the subwoofer... you need to verify with manufacturer on what that speaker needs... certain sized enclosure, sealed or ported...

Whether you are going to use both of them at once or just one... that speaker is designed to be used both at the same time. If wired parallel, it will yield 3 ohm... which is kinda high load. 6 ohm speakers are usually used for home speakers.

Your amp, on the other hand... you need to find spec on it... when you do bridging... meaning 4 channels into 2 channels, it will only allow 4 ohm or higher... going lower will mean amp internally melting and die. If you hook up the speaker... one to each channel, then you are fine... But you will have to see if the sound is good... Because the speaker at 6 ohm will draw lower power from amp... may sound a bit weak.