There are some compatibility problems with the fish you name.
Platies are hard water fish while the others are soft water.
German rams need water warmer than most fish, and certainly warmer than cories would be happy with.
You would need to buy a bonded pair of rams as just any two fish could well end up fighting. And if they bred there would be problems with cories which have no concept of territiory and would never learn to stay out of the rams territory. You could end up with beaten up cories.
300 gallons per hour in a 20 gallon tank is far too much water flow for gouramis which like slow moving water. The aquaclear 50 is rated for a 50 gallon tank. The Aqueon Quietflow 10 would be sufficient for honey gouramis on its own.
To be honest, 300 gph is too much for any fish in 20 gallons except maybe hillstream loaches. That's 15 times turnover per hour.
We need to know how hard your water is before we can make further suggestions. Your water supplier's website might give your tap water hardness on their website; if they don't, take a sample of tap water to a shop and ask them to test for hardness - but make sure they give you the number and the unit.
