You'll have to see how your body does with natural sugars such as honey, molasses, etc. After completing the plan, people are much more aware of noticing negative changes in their overall health when adding in sugars - we get hundreds of e-mails sharing this experience.
The problem with sugar substitutes is that while refined sugar (sucrose) may be more toxic, in many ways, sugar is sugar when it comes to how the body responds (with the production of insulin, inflammation, hormonal responses, etc). Excess insulin promotes inflammation and blood sugar imbalances. All common sugar substitutes, including Stevia, in a sense, are "good" enough to fool the body into reacting to the "presence" of sugar, which can again initiate the production of insulin and other changes.
Also, people tend to eat more sugar when they are defining it as "natural", thinking that there is no negative effect or downside. Sparingly is probably the best way to use additional sugars of any kind, if at all, when adding them back into your diet. Blackstrap molasses, agave nectar, and honey do have beneficial properties, but overdoing it can have negative results. You will need to see how your body does - and having completed the plan, you'll probably be more aware of your body's responses. The only sugar that I would ever recommend would be raw, UNHEATED honey, and then only if it works for your body.
Synthetic sugars like aspartame, sucralose, and others have been found to produce very negative effects on the body's nervous, immune, hormonal, and enzyme systems. Acesulfame-K, otherwise known as acesulfame-K, or Ace-K, has never been shown to be safe for human consumption. It is suspected of being carcinogenic. It's probably best to avoid synthetic/artificial sugars.