You really don't have room for any more fish .... Four 3" golds is too much for the 10, never mind the Plec & snails.
What are your parameters? I would think that you would be needing large & frequent water changes to keep things under control. If you have diatoms, they should be easy to vacuum up. So just vacuum them up with your
pwc's. <Diatoms are mostly found in new tanks, should go away after a while.>
In the long term, you would need a bigger setup for your critters.