If you're running the additional speakers on their own amp than you can go with 4 ohm, but if you're going to install an 8 speaker system I'd upgrade everything and go 4 ohm all the way around and find big power to push it with. Looking at the specs for that amp (assuming you'd use the same one) it only pushes 100w at 4ohm.
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2018 CVO RG lava orange, 117 Stage 3, 14" F47 and a few chrome extras. Boom crap gone.
2016 RG Special Cosmic Blue, 120ST with Dragos 2:1 and TTS tuner, FBI drop seat kit and JNR air ride. Stock audio long gone.