smaller tanks are more sensitive to water quality. strange as it may sound, bigger tanks are actually easier to maintain and are more forgiving in water chemistry.
so please consider getting a larger tank if you want to keep angels. a 25 gallon tank wouldn't be all that much larger in footprint, 24x12 versus the 10 gallon being 20x10.
otherwise, for your current 10, several more tiger barbs are all you'd want to keep.