We typically create a separate group within the class. Once you have a group you can use the editor to make assignments visible to one group but not to another. You can also change other settings for individual groups like time allowed, number of attempts, etc.
The group manager can be found in the people tab under the wrench at the top.
Agilix has a page here discussing group management.
Thank you for explaining that you can:
"make assignments visible to one group".
How do you do that?
First, open the activity in the editor.
Second, under visibility and settings set the visibility to teachers only.
Third, click on the person icon in the upper right. A pop-up will open showing your list of groups. Select the group you made for your special needs students.
Lastly, you go to the visibility settings and set it to students, teachers and observers. Now this activity is only visible to this group.