1) Breastmilk is always more nutritious than cow’s milk, but breastmilk alone is not sufficient for your toddler’s nutritional requirement. You need to give a well balanced diet. So, no.. you don’t need to top up with cow’s milk if you have enough breastmilk.
2) Some people definitely do get pregnant while breastfeeding, although if a mummy breastfeeds very regularly, sometimes it may act like a form of contraception. So I think you can try doing both breastfeeding and trying to get pregnant, but if it’s hard to get pregnant after some time, then you may want to think about stopping breastfeeding.