I have a few lead gen sites in the same niche, on the same server. Should I create unique designs for each to avoid any issues with Google?
It is recommended to avoid using the same design as it may leave footprints. Have each design be unique, with unique images, a unique number of pages, different topics, different linking structures, plugin configurations, everything as random as possible.
Also, if you are using shared hosting, avoid linking between these sites. If you are linking between sites, then make sure you put them in different hostings (sites should have different IPs).