Wouldn't you have to sweep from the guard into the ferret, since that is the only way you can attack the fuzzball? It's mandatory to attack it, and this is a way to do so, after all.
Before you decide what to do with your action, you check to see if you are required to use it a certain way due to mandatory actions. You are only required to do a mandatory action if able, and more importantly you are never required to make yourself able to do a mandatory action. This is in the rule supplement.
Thus, since there is a guard you are unable to attack the ferret and you are not required to get rid of the guard to make yourself able to attack the ferret. You are therefore free of your mandatory action and may take whatever action you wish.
If you choose to attack the guard and you get a secondary attack due to sweeping, battle furry, etc, you are not required to use that secondary attack on the ferret because it's part of the same attack action and you only check for mandatory actions before you take your action, not in the middle of it.
@bigfatchef
Yes, you choose the second target during the second declare attack step. They are two attacks and you don't have to make any choices about the second attack until you get to it. However they are a single action, which is why the taunt doesn't kick in in the middle. The difference between attacks and attack actions can be confusing. But you could choose to attack the ferret, another creature, or nobody with the second attack.
