The media in the canister filter will also become a nitrate trap...along with those bio-balls. Clean it often. As for the rock, it'll become live rock soon enough. Keep in mind that your tank has been cycling for 3 weeks now. Adding that new live rock might start a mini cycle again. That's alright though, you need more live rock.
As for the 29 as QT. It'll be fine for just about anything you get, fish wise. If you plan to QT "everything wet," which is a good idea, you'll need to eventually consider upgrading the lighting on your QT tank. To turn it into a SW QT tank, you will need to totally clean it, remove all FW substrate and start it as a SW tank. As for cycling it, put a sponge filter in your 75 gallon's sump/wetdry. Let it colonize with good bacteria and then put it in the 29. That'll help cycle the 29. Get rid of the biowheel but still use the penguin for flow. Here again, the biowheel will contribute to nitrates. As for Algae to put in your sump, go with Chaeto. You asked how many pounds. You won't need much. Probably won't even weigh a pound. Get a quart sized bag of it. It should be loaded with pods and beneficial critters to seed your tank with. Give it light and flow and it'll grow. It has to stay submerged and needs enough flow to keep it in a slowly spinning ball.
As for Tangs. I will again, STRONGLY, recommend that you don't even think about more than 1 tang in a 75 gallon tank. It's not near enough room. Even with only 1 tang, there's only a few species that are small enough for a 75. Yellow Tang, Scopas Tang, Regal Tang and maybe a Purple Tang. Everything else is going to get WAY too large and requires WAY more space. Most of the larger tangs should be in a tank no smaller than 200 gallons, IMO.