I have a web application with n number of site collections and each site collection has a custom Task list and other list. Is it possible to create a search topology to index only the custom Task list and create a Content Search Web Part connected to this index?
A separate SSA would be overkill (IMHO). If I were you I'd create a Result Source for that list and then bind the search result to the Result Source.
In SharePoint, You can use the Content Search web part and "contentclass:STS_List_Task" query to get all the Task lists. You can follow below steps:
Edit the page that you want to display content and select add a web part. Choose and add a Content Search Web Part.
Edit the web part, click “change query” button and specify you query.
Switch to advanced mode, in “query text” region, enter the following text:
"contentclass:STS_List_Task" is used to filter by task lists only.
For more information about SP search using contentclass: