I would advise against an RO filter. They are very slow and they take out all the good minerals. I use a Kold Sterile filter. Passes 4 GPM and is good for about 15,000 gallons before you need to change the filter elements.
If you have well water, you probably don't need it. I didn't realize these guys were in a 10 gallon tank. The ammonia can spike quickly in such a small volume.
I would set up the 100 gallon system and put the fish directly in that system and seed it with more aquarium gook. The large water volume will bugger the ammonia/nitrite buildup and give the bacteria time to grow. Start with maybe 20 fish and once the tank is stable in 3-4 weeks, you can add more fish.