Yes, you are correct clear quartz does amplify other crystals. So, using it with others can be helpful. You can use crystals in many ways. You can place them in your home and just feel their energies. You can hold them for several minutes or as long as you want and feel their energy. You can also meditate with them by holding them or laying them on your body. Crystals are very healing.
I don’t think you can affect them with negative energy, if anything the crystals will help decrease, shield from or take away any negative energy. Some crystals that do this are black obsidian, black tourmaline…
Rose quartz is great for the heart and heart chakra. It can bring love to you if you carry it with you or just have it in your home.
I hope this helps some… Feel free to ask some more.