I almost never use Beersmith's estimated FG. This tends to be difficult to predict as there are a number of factors that affect this. Things like the amount of crystal malts used, maltodextrin, mash temp, yeast attenuation and pitching rate.
Keep in mind, if you want to keep sweetness after fruit additions, there are additives like potassium sorbate that inhibit yeast, so you can use that to keep fruity sweetness if you wish.
Also keep in mind that the body of your beer will not necessarily change by the addition of fruit. You will still have an equal amount of unfermented maltose before fruit additions as after, assuming you allowed for a full fermentation before secondary.