1. You need to consult the guide for your computer or motherboard for the correct order to install memory in the slots. If you are buying new memory you should just opt for a single 4GB stick so it is paired with your other 4GB. For most systems this will give you the best performance and allows the best path for future upgrades. If you are installing the 2GBx2 because you have them sitting around, then most times you can mix them, but one will probably go next to the existing 4GB stick and the other 2GB will go into the first slot on "the other side" as a general rule. Your system guide should tell you if the 2x2GB should be paired in the first slots or whether the larger stick should be there. If you purchase just a single 4GB stick, it will usually be installed in the matching slot "on the other side" as you put it. I'm assuming a standard Desktop PC with a single CPU board.
2. The "L" in the name means "low profile" so the memory stick is usually shorter and in most cases should work fine. The "1.35V" should be Ok in most cases as well especially if Crucial lists it for your system. It just means the new memory requires less voltage to operate.
3. Use the Crucial website to identify your system or motherboard, or use the Crucial System Scanner to identify the memory you should purchase. If you use either of the those options, I believe Crucial will guarantee compatibility.