what is the best way to add user access from child groups to parent group (large enterprise)

We have investigated the steps for denesting of AD groups. 

Step1: here we are extracting the reports of nested groups 

Step2: we are going to add the groups from child AD groups to parent AD groups.

Step3 : we will be denesting the Nested AD groups and make all the groups independent. 

My doubt is when we are doing the process of adding the users directly to parent group what challenges do we face. If anybody has already done this process. please guide

1. do we get issue in adding users directly depending on group scope

2. does this have any limitation in numbers when we add access directly from child group to parent group (usersC will be added UsersB and then UsersB & UsersC will be added to Group A

NEsted group:  Group A: usersA

                                   Group B: usersB

                                                 Group c: usersC

suresh arasu