My son is 8.5 months now and at 7 months he would eat about 2 tbsp of infant cereal for an additional source of iron and then 2 ounces of fruits or vegetables. Since we were following the 4 day rule and feel that breast is best, we tried to not put too much emphasis on solid food as it was still relatively new to him. I allow him to breast feed on demand, but when I know that I want him to have solids - I will wait an hour after he last nursed. I did some research because I had some concerns too (like you) and some babies will continue to eat solids after they are full if they enjoy the socialization of feeding time with mommy. You can safely feed your 7 month old 2 to 3 ounces of solid food twice/three times a day. It's up to you and your baby. My DS will nurse first thing in the morning and about an hour after waking, then after each meal and before each nap, then before bedtime and through the night to help him sleep.
Hope this helps!