It varies by the tank really. This is just one of those things where it depends on the situation. Usually people try to get a freshwater aquariums somewhere around 10x to ensure that the water is getting around sufficiently. You can get by with less as long as there is good circulation and not a heavy bioload, and you can do fine with more of the fish like the current. A Magnum 350 and a AC500 should be good for your setup. I would put the AC500 in the middle, and the Magnum intake and outlet hoses on opposite sides of the tank. Adding another filter later is quite easy, if you feel the combination is not enough.
Having extra filtration it just one of those things that does not hurt, and can help quite a bit to keep the water clear and safe for a long time.
2x Sunburst Platys (2M, 2F) // 3x Neon Tetras (?) // 2x Cherry Barbs (1M, 1F) // 2x YoYo Loaches (?) // 1x Bushy Nose Pleco (M) // 1x Clown Pleco (?) //
-30# Live Rock
-Mixed Snails (Nassarius // Cerith // Astrea // Black Turbo // Mexican Turbo)
-8x Blue Leg Hermit Crabs // 1x Banded Serpent Star // 1x Cleaner Shrimp
-1x Royal Gramma // 1x Percula Clownfish
-2x Mushroom Leather // Metallic Green Star Polyps // Pulsing Xenia // Striped Xenia // Blue Mushroom
-Remora + Maxi-Jet 1200 // 2x Maxi-Jet 600 // Maxi-Jet 400 // Visi-Therm Deluxe 50 // Current USA Orbit 2x 40w PC