It’s ok to spread yourself into many niches, but keep them all related to the same top level category. That means a broad horizontal niche like health and fitness could cover vertical niches like gym memberships, supplements, home workouts, yoga, treadmills and bottled water.
Your list management software can keep each audience segregated for targeted advertising by using tracking codes. But the real secret lies in having one generic message to keep in touch with them all.
That means you can have one set of health and fitness autoresponders, articles and newsletters that you send to everyone. Then when you want to sell to one group, use your mailer to target by tracking code.
And here’s the unadvertised bonus. It’s also better for SEO when you link related sites together. The links will be considered higher quality, given more weight, and contribute to higher rankings.