Isabel, speakers from different manufacturers can certainly be combined, but it's especially important that the three front speakers(if you use a center)should match closely in their tonal character. The simplest way to do this is to use three identical or closely similar speakers of the same brand.
The pre-amplifier processes the low-level signals from a CD, FM tuner, etc. and the amplifier then makes them strong enough to drive the speakers at loud levels. A receiver such as your 1400 is a combination of a tuner, pre-amplifier and amplifier, so you already have all this. Some choose to hook up their receivers to separate amplifiers in the often misguided belief that they'll get better sound, but this is seldom necessary.
