I would wait to get the corys, they do better with a more mature tank IMO. BN is an omnivore & can help with uneaten food not just algae. I'd probably start with rasboras or a BN. How many guppies do you have now? 1 BN will be ok now especially if it's small. If you have, say 8 guppies, I'd remove them all & add all the rasboras all at 1 time.
Each time you add fish you risk diseases, so study the lfs tanks for anything that looks "off", skinny or any white dots or other skin or fin issues. On a communal filtration system?, then study all tanks! If your cousin can go with you great!, an extra pair of eyes & some experience can help choosing fish. If the fish don't look really good, wait. I know you're ready but get the healthiest fish you can.
Test daily & be prepared to do a large water change as needed. Prime can also help with any ammonia or nitrite if you "really can't" change water. Any of either can cause permanent damage if present for long. Prime can "bind" both for 24-48 hours, dose for entire tank volume if needed but test, test, test!