The Email Test Group will handle the sample.
However, if you really only want to add 10% of the entire list, please use Random Sample. Well documented here.
Not sure if you were trying to link something in your last comment, I would appreciate the information.
I'm planning on adding the Salesforce leads into a static list, which would be the first step.
After that, I'm creating this smart campaign, but for some reason the batch campaign is saying that all 1,500 leads will be affected. Should I be seeing only the amount of leads who are going to be added?
The number of leads that will be affected is calculated based on the Smart List. It doesn't know that only 10% will actually go through the flow.