Just keep all tanks
Maybe you could look at adding some other cold water fish to your goldie and putting it into your 3ft tank
Set up a community of tropicals - not sure about the silver shark - don't they get big?? if so you might need just the couple of fish you already have to leave room for him to grow!!
I put enough gravel in to bury the plants in - its difficult to say an exact measure - mine also slopes from back to front ie more at the back and less at the front - but if you are adding caves, wood, plants, ornaments etc it will vary
I'd keep your elite and use it as a hospital tank / quarantine tank /fry tank