You can add a 2ch, 4ch or multiple amps. As long as you have at least one set of rca preamp outs, you can split the signal to as many amps as you want.
If you're going to be running all 4 stock speaker locations from a 4ch amp and a sub from a bridged 2ch or a mono amp you will probably have built in crossovers in the amps to ensure the mids/highs don't receive any bass signals that will make them try to reproduce frequencies they're not designed for.
You could also run a 4ch amp (as long as it's bridgeable to 3 channel mode) for all of your speakers.
Running the mids/highs on ch 1 & 2 while bridging ch 3&4 mono for a sub. I did this for years in my truck & it worked like a charm.