Brief Answer:
Ideally complementary feed should be started after completion of 6 months

Detailed Answer:
Hello

Thanks for asking on this platform

Exclusive breast feed should be given till 6 months of age.
There are many reasons to start complementary feed after completion of 6 months.
Mother's milk production peaks at 5-6 months of age.After that baby needs extra amount of calorie and fluids which is not met by breast feed only.
Also baby starts mouthing(try to keep all the things in his mouth) and teeth starts erupting.
One should start gradually beginning from well cooked one cereal in small amount.I usually prefer home made food instead of formula feed(which is also made up of cereals but contains preservatives).
So I would suggest you to start with cooked rice powder or boiled mashed rice in milk.You can add small amount of sugar to add the flavor.Extra salt should be avoided till one year of age as baby's kidney is not mature enough to handle electrolytes.
You can then give ragi,pulses(khichdi made up of rice,pulses).
Add wheat based diet at age of 7 month.
The reason behind adding cereal or any food item one by one is to catch easily if there is intolerance to a particular item.
You can give suji ka halwa,rice with milk/dal,roti mashed in milk/dal.
Also give mashed boiled apple/banana/boiled potato/chiku to meet the demand of vitamins.
Start with few spoons.Then gradually increase the amount.By 8 months of age the baby should get 3 major meals(each consisting 3/4 to one small katori) made up of cereals mixed with dal,vegetables and 2-3 snacks made of fruits,biscuits,bread etc.
Continue breast feed 3-5 times a day mainly during night time.
One should avoid cow's milk(if not taken earlier) and egg white till one year of age.
Monitor growth of baby by serial weight record.
For vomiting you can give domperidone drops(5 mg/5ml).The dose is about one drop per kg.If baby's stool frequency gets increased you just monitor the urine output and oral acceptance.Most of the cases of increased frequency of stool don't need any specific treatment.Continue giving vitamin drops as usual.

I hope this helps.
Please let me know if you have any more query.
Regards

---