Your baby can take custard with age appropriate milk.

Yes, you can add it to the pap.

Remember: Mealtimes are about more than food. They are a time to connect with your child and to support her overall development. Talk with your child during meals and don’t let her eat alone. This helps build strong family relationships. Also, Create routines around mealtime. Routines make children feel loved and secure. Establish regular meal and snack times beginning when your child is 9-12 months old. Routines help children look forward to each meal.

Yes you can give your baby custard.

Babies crawl when they are ready averagely 9 months Some don't even crawl..

Heinz Fruity Custard, Hollandia Custard etc

I started giving my son checkers custard since 4 months

Not Recommended because Custard is a very rich food, and is high in fat. Your baby doesn't need to eat a lot of fatty foods at such a young age, and you are not doing them any favours by introducing this at this time. You want to wait until your baby is more active, crawling around or after they've learned to walk.