You can't keep them in a twenty gallon tank, it is far too small. Therefore, I won't give you an answer unless you have another tank to accomodate them (if there are others than those in your sig).
Black widows are not good for cycling a tank, as they are fragile. Also, they need to be kept in groups, and a 20 gallon tank might not be enough.
4 goldfish in a 10 gallon? They are filthy and therefore require a lot of care. Also, they require more oxygen than tropicals and therefore greater surface area. Seeing as how this can be a problem in a small tank, I would suggets finding ways to remedy this.