Before answering your questions, I need to advise you to ignore the shops. Most of them don't have a clue about setting tanks up properly. And without intending any disrespect, ignore your father too. Fishkeeping knowledge has moved on enormously in the last couple of decades.
Firstly, you say you let the tank cycle for a week. I assume you mean you let the tank run but didn't add anything? In which case, it was not cycled and you are now doing a fish-in cycle.
You say you have been monitoring the water closely - have you been measuring the ammonia and nitrite levels? If you have, good
And have you had any reading above zero for either of them - and doing water changes as and when necessary to keep them both below 0.25?
Q1.
You can add more fish when both ammonia and nitrite stay at zero without you needing to do water changes. There is no set time, it depends how long the filter bacteria take to grow. Once you've had double zeros for a week, you can get more fish - around a third of the fish in there now. If you get too many at once, the bacteria you have grown will be overwhelmed. Since you have 3 danios and 3 mollies and you want more danios, given that mollies (even baby ones) are bigger than danios, you can get 3 more danios.
Q2.
Do you want a fish that feeds off the bottom or a fish that eats algae? If it's just fish that live on the bottom, why not look at corydoras? A tank with a footprint the size of yours will need one of the dwarf species of cory rather than one of the larger species - that's
pygmy cories,
salt & pepper cories (not peppered, cories they get too big) and
hastatus cories. They need to be in a shoal of at least 6 of the same species. They are not vegetarians despite being a type of catfish; they need to be fed on sinking pellets that contains some meaty ingredients.
If you want something that eats algae, the best creature for that is nerite snails.
Q3.
Weekly water changes of 25 to 30% is the best regime. But that's only once the tank is cycled - once both ammonia and nitrite stay at zero by themselves. Until then, water changes should be as often and as big as necessary to prevent either going over 0.25.
Q4.
With just 24 inches length, you can't have very big fish. They need more swimming room than you have.
Do you know how hard your water is? A rough guideline is if your kettle or shower head fur up, you have hard water. Your water company should have the hardness somewhere on its website. If you find it, can you give us the number and also which units they use. There are several of them (like inches and cm for length).
It is easier to suggest fish once we know what your water is like. Hardness is more important than pH. You have some time yet before getting all your fish