I have one questions. When parents/spouse immigrate to USA with IR-5, CR-1 Immigrant Visas, what form do they need to fill out before landing to USA? I know, custom form is a must, but how about I-94? Are there any more forms that they need to fill out? Please reply in general about what kind of forms one has to fill out when coming with an immigrant visa.

I believe that the customs form is the only form that is required for someone using an immigrant visa. The I-94 is an entrance-exit visa and is for non-immigrants, not imigrants. I am sure someone else who has entered on a CR/IR will be able to confirm this.

Kathryn is correct. If you read the instructions at the top of the I-94, you'll notice it says that aliens with an immigrant visa do not need to fill out the I-94. If you have an immigrant visa, the customs form is the only form to fill out on the plane. Only one customs form is required per family travelling together.

I agree the two posted above is right. I entered here as IR-1 too.

Here's a thread about coming through the POE with an immigrant visa:

Arriving in America, The POE Drill

All would apply to parents as well as spouses.