Do I need to combine protein sources at each meal to make sure I get all essential amino acids?
No. It's a myth. Nearly all whole foods (including greens and veggies) contain protein, and nearly all forms of protein contain all protein-forming amino acids in some quantity. If you eat a variety of nutrient-rich, whole foods, your body will work its magic to store what it needs to function optimally. Scientists used to claim that herbivores would develop protein deficiency if they didn't get essential amino acids in proper, combined amounts at every meal. But our bodies are amazing. See, when we eat, amino acid "deposits" are made into a "storage bank," and the body then takes what it needs as it's needed. So, you don't have to eat complementary proteins together with each meal in an effort to make complete protein. Your body has a system already set up; you just need to eat a variety of foods to fill the bank. By eating a variety of plant foods with "incomplete proteins" throughout the day, we can easily get enough "complete protein."
Picture a giant 400lb gorilla. What does he eat to grow that strong, muscular body? Leaves, stems, roots, seeds and fruit (and maybe the occasional insect that lives on those foods). The body actually works less (uses less energy) to process protein from a vegetable-based diet than it does from a meat-based diet, too. This energy conservation is great for more efficient healing, building and overall optimal health. Most plant-based protein sources, contrary to animal-based sources, are also alkalizing for the body. Which means less aches and pains, freedom from disease, better sleep, happier moods, improved memory and concentration, stronger/more efficient digestion and an overall better state of health.