You haven't given size of skimmer or waterfall units but, yes goldfish will be fine in that space. Do yourself a favor and put in a BD even if you think it's only for cleaning it out once a year— the cost is cheap, relatively, and once the pond is built it's a tough one to redo. Once the plumbing is in place, it's there when you decide you DO want to use it. Many people say go bigger because once people build their pond they generally want it to be bigger. A 1200–1500 gallon pond is certainly a fair amount of water for GF to swim about in. Most say that the more water there is the easier it is to take care of—temperature fluctuation as well as volume of water for dilution of fish waste, etc. A one foot deep pond is going to freeze and boil quicker than a three foot pond. If you know the perimeter size and it fits well in your space you just need to decide on a depth that will work for your situation.
Once the pond is in place a few years one can have a different reality than first conceived, when it was just an idea. My wife thought she was loosing all that planting garden space to a water hole. Now she can appreciate that the garden was really expanded not contracted.
You have to take in everyones ideas then form one of your own. Most folks are trying to help you avoid the mistakes they made.