Hey I dont often contribute to the forum. I use to keep goldfish at my house for my mom till I left for college and she neglected the tank. However, you really shouldn't keep neon's and goldfish together. Neons are tropical loving fish and like water 75 degrees or higher, goldfish can tolerate 75 degree water, but it would be better if it was around 68 or 70. However, I would say one goldfish is okay in your tank. However, goldfish are very dirty, so I would change the filter at least every 3 weeks and do weekly water changes and Bimonthly cleaning. I had 4 fancy goldfish in a 30 gallon tank. Indeed it was overstocked, but I changed the filter often, had it aerated, my weekly cleaning I always used the siphon, and did bimonthly cleaning. They did fine till I left for college and the died about a year later
Basically, if your willing to take extra care of him then great, but if you find it too hard, then upgrade and he will be just fine and happier too! Also take the neon's back.