Agree with previous posters. Your lone cory must be very lonely, they are most assuredly a schooling fish and always do better (health, temperament, activity) in groups of at least 6. You can mix species or stay with one species. I have found that when mixing species there seems to be more interaction if there are 2 or 3 of each species. I have over 15 years kept some of the same species in both situations and this is always evident. So 3 of 1 and 3 of another, or 2 of three different species would work well if you want variety.
For algae control, if you have plants (and I assume you will for the angels), the otocinclus cats are the best, and will fit in nicely with your cories and angelfish. Farlowella also are avid algae eaters in a planted tank, but this is not a fish for everyone. And in either case I would recommend that you wait for the algae to begin showing before you add the otos or farlowella; they won't fare well in a new tank that is free of algae.