My pediatrician suggested starting to introduce her to foods around 4 months so I started then. She's 6 months old now and hasn't had any issues. I use the jar food and sometimes I make my own. It just depends. If I use the jar foods, I am very cautious about what's in the ingredients. Sometimes I use those organic squeeze packets. They just have the food and a little water in them.
mixed ones? like mixed fruits and veggie. I wouldn't start a baby off with mixed one only because you want to identify if your baby is sensitive to certain foods.