You need to consider the hygiene of your baby. Especially where he/she sleeps. Ensure it's very clean. I've used cacatin cream in the past and it worked well

No, it is not. Those drugs are corticosteroids. Prolong use of them have side effects you won't want to be dealing with later.

If you want to add weight in a healthy way, eat a balanced diet that contains healthy fat, take supplements and rest well.