Switching a stereo source to mono usually sounds pretty flat and hard to engage with to me.
The electrical parts of this are fairly easy so no reason to buy a different amp just for more outlets. However the big issue is the impedance. Most bargain amps only do 4 Ohms, so you can only put 8 Ohm pairs in parallel.
Another alternative is to get a semi-pro/installer grade amp like the Parasound Zonemaster amps which are ideal for this type of setup.