That's not how segmentations work. The Segmentation does cover your whole database.
If you want to restrict the send, just use Member of Segment= to restrict your smart list.
Let's say you had Products A and B. Each Product has 200 people who are intersted in it. The other 9600 people have not stated an interest or an inferred interest.
These would be your Segments
Product A=200
Product B=200
Default (everyone else)=9600
If people in Default identify themselves as A or B, Marketo will automatically move them over.
When you run a campaign, you can do a few things:
Run 1: 10,000 people, Emails with A and B info get sent to A and B segments. 9600 people get Default.
Run 2: 400 people, because you selected Segments A and B in the smart list. Email goes out to 400 people and A/B segments get their emails.
Run 3: some mixture where 250 people in Segments A and B get their emails. And 300 people get the default.