Yes, they work great with freshwater tanks.1. Are sumps okay for freshwater?
Biggest you can fit in the stand, generally. If you can use a 30, I'd go with that. A 20H would be my next choice. I don't think I'd go smaller. A 30 regular, not a breeder would be ideal. Many use the rubbermaid tubs. Probably a better option as they're cheap and more sizes are available.2.If I were to set up a sump on my 55 how big if a tank should I use?
Generally, some type of prefilter pad is put in place to catch large particles before the water runs through some type of biomedia. Many types of arrangements can work. Many possiblities. The DIY forum has many of these examples. I'd do a search on that and see what others have done.3.What is put in the bottom tank to filter the water?
Not worth the bother IME. Better to consider using regular house plants with the roots submerged, if you want a nitrate reducer. Easier to do, and inexpensive to try. I've not found it necessary. I just do the water changes.4. I have heard of planted sumps to reduce nitrates, Is these easy enough to do with a noob like me?
I like the mag drives. You want a pump rated to push about 600gph at 4' head. A mag9.5 is rated at 800gph at 4' head. Better to have too much as you can always throttle it back. Assume total volume with sump at about 75gallons, so about 10X turnover. More than you need. 6X works well for me on my systems. I have 9 tanks running with this rate of turnover. You don't need more. You need just enough for biofiltration. These systems oxygenate water very, very well. If you go smaller, you generally don't save that much.5. My 55 is on a stand , total height of tank and stand is 48.5", what pump do you reccomend?
I've only used drilled tanks, so I'll let someone else chime in on that.6.Saw a thread on a pvc overflow design, could this be worked in to be cheaper?