Q1: Connect IPs? Each droplet already comes w/one IP;
Q2: None (why add another layer of complexity; and you have more granular control over your droplet with the command line). If you must have one, then I'd suggest Webmin/Virtualmin;
Q3: No;
Q4: Yes, but you might not want to if either of 'em gets too big. However, you can always start on the same server and transfer one installation to another if needed.

Thanks!

To add to this:

Q1: I have three sites with SSLs running from the one box (1 IP).
Q2: Yeah, don't bother.... removes resource from running sites. Learn... everything you need is in the help sections.
Q4: I have 2 Magento sites and 6 WP sites running from the same $5 box. Obviously low traffic but you can't beat the value.

(LEMP stack)