The rule of thumb I was told was around 6 months... unless the baby is ready before. The way to tell if your baby is ready is if and only if - they can hold their head up without support, have doubled their birth weight, and show interest in what you are doing when you eat (moving mouth when you choo or opening their mouth when you put a bit in your mouth). I don't remember how old my oldest was, but my 5 yr old was about 8 mo, my 4 yr old was about 7 mo and my 2 yr old was about 3 mo when we started giving them cereal & small amounts of baby food. The reason they all started at different times was they all reached the milemarker (rule of tumb) at different times... my 2 yr old was 8# at birth and 16# by the time he was just over 2 mo old.
Honestly... breastmilk is all they really need for the first year. If you choose not to give anything else, it won't hurt them. But I do use the other stuff as well... but remember, if you do choose to give your baby the cereal or baby food, only give her one new item every 5 days or so. That way you are able to watch for allergies... if she does have a reaction, you will know what she is reacting to if you give her the 5 day window. And when you first start giving cereal, it is almost soup... super runny (I mixed mine w/ breastmilk), but use a spoon to feed her with - I never put cereal in a bottle.
As for supplementing breastmilk w/ formula - I did it with all my kids from birth. You see I have family member who don't believe in breastfeeding, so if they are around I respect them & give a bottle of formula. My work never really gave me a hassle about pumping (well except my boss - she didn't believe in nursing), but I had to do it in the lady's restroom & would store the milk in the employee freezer. No one ever said anything as long as my work load got done... with the expetion of my boss's personal input about breastfeeding being a waste of time & that I just plain out had to many kids. There are laws that protect you as a nursing mom - that let you pump at work without any issues. As for your mom, I hope she will be understanding to your situation & know she will love you for doing all that you can for your baby.
You have already given your baby the best start you can, and have shown her so much love... just keep up what you are doing. Remember when it come to your little one "mother (you) knows what is best". Take care & congratz on you baby.