What kind of car do you have? Its fine to mix brands, I don't think there are many people here that only run one specific brand throughout their car. If they do, it usually seems to be higher quality, or more expensive/overrated stuff. The shops have a good point about having room to upgrade in the future, but they also want to sell you more equipment. Many people here will also say that you only need to power your front speakers and run the rear speakers off the H/U power. If you do this then you can get away with a four channel amp, using two channels for the front speakers and bridge the other 2 channels for you sub. You can always mount the amp on the box, build an amp rack, put it under a seat if it is small enough, etc.