I would tend to assume that most generic (or cheap) subs would be 4 or 8-ohm. However, you don't want to risk your amp on an assumption. If you have a multimeter you can read the impedance with that.
If you wire the two 6-ohm subs in parallel (+ to +, - to -) the amp will see a single 3-ohm load. If the other subs are 4 ohms each, and you parallel wired them, the amp would see a 2-ohm load. In this case, a good 2 channel amp would power all four of them nicely. Run the left channel + and - to one of the 6-ohm subs. Then connect the + of that sub to the + of the other 6-ohm sub, and the - to -. Use the right channel of the amp to power the other two subs, wired in the same way.
As far as the crossovers go, you won't want those Cerwin Vega units. They are for running a midrange and tweeter, not full range and subs. Passive crossovers are cheap, and you should probably cut out anything below 100hz or so on the front and rear speakers. Let the subs handle everything from 150hz or so, on down. These numbers are flexible... some people make the divide around 200hz or so. Most amps will have a low-pass crossover so you can cut out the high frequencies to the subs. Then all you'll need is some "bass blockers" for the front and rear speakers.
Bass Blockers at Crutchfield
How much are you looking to spend on an amp?