I wouldn't fill it with anything less than 200 liters. Simply, the headspace will become problematic on a number of levels.
If you're making beer to sell, then make enough to sell. Your time is better spent that way and your business will benefit more from selling than testing.
It should only take a batch or two to learn your beer system well enough to predict how a recipe will turn out. You should already have a very solid knowledge base about your ingredients and techniques. I always recommend doing a very basic wheat beer or pale ale, that has a wide target range. No matter what balance it has, it'll sell as long as it isn't contaminated. Once you taste a basic beer and know how it meets your expectations, you should be able to hit a full size batch of anything and get close to perfect on the first try.
All things considered, if you need to "test" recipes, you should have a smaller pilot system. You shouldn't have any fear about making a full size batch.