If you are planning on using only the H/U's onboard amp, then you should use whatever they recommend. Most are in the 4 ohm range. My statement about mixing was regarding channel pairs. For instance, not putting a 2-ohm speaker on the LF and a 4-ohm speaker on the RF. Generally, using two 2-ohm speakers for the front and 4-ohm speakers for the rear (or vice versa) should be OK, but it depends on the H/U. Most prefer the use of matched speakers all around. External amps are a little more forgiving so long as they are rated for 2 and 4 ohm operation.

 

But you do change the amperage and wattage of and amp when you reduce the ohms load the amp sees it increases the output wattage.....This is the difference between a crappy amp and better amps....If you wire a good amp up in a 2 ohm load it theoretically doubles the rated output of the amp....it also draws more amps and runs hotter.....this is the pricipal in some amps designs like the 25watt x2 (in a 4ohm load) orion HCCA in my vette powering my subs....I have two dual voice coil subs wires in parallel and then parallel again which puts a 1/2ohm load on the amp and icreases the rms wattage output to 200 watts per voicecoil .... The amp Im putting in the ram will push over 600 watts rms in the same load but overheats if I don't provide additional cooling and I had to beef up the power supply wiring as it would distort blow the fuse because of the additional power draw in my jeep before I beefed it up and added a cap....
Good amps are 1 ohm stable bridged decent amps are 2ohms stable bridged....I run mine at 1/2 ohm and they really cook and need additional fan cooling.
most cheap amps are 2 ohm stable not bridged and 4 ohms stable bridged.