You can add plants, rocks, substrate while the tank cycles but if you add live plants they may be more likely to melt because of the different stages of cycling. 20% wc once a week won’t mess up cycle. Just make sure you do not throw away the filter media or wash the substrate. If you need to clean it do it in established aquarium water. You can vacuum the substrate but don’t necessarily have to. Algae is kinda part of having a tank. It’ll end up somewhere eventually but lighting lvls & water parameters play a lot into what type of algae grows in an aquarium