Dedicated IP + Multisite + Web Hosting

Hi. I have VPS and few dedicated IP. Currently my dedicated IP (example) is as below.

1) 123.456.789.123 - VPS Server
2) 123.456.789.456 - Main Domain Website abc.com with SSL
3) 123.456.789.789 - Empty (Unused)

*Note: My WP + WHMCS will be installed on my main domain with SSL

My website have 2 purpose. One is to sell web hosting (cpanel) and one is to create multisite (instant website for businesses). Some client might just want to buy cpanel/web hosting. but some might want to get instant website and cpanel/web hosting. All my multisite will use domain mapping plugin from WPMUDEV and will map their site to their own domain.

My Main Multisite abc.com is hosted at IP (2). So if i create site for my clients, what is their IP will be? I presume they share the same IP as (2). Their site would be abc.com/user1 and so on. If they share the same IP with the main site, will it affect my domain acb.com / ip if they does a bad website? spam etc? i was wondering on doing like this:

1) 123.456.789.123 - VPS Server
2) 123.456.789.456 - Main Domain Website abc.com with SSL
3) 123.456.789.789 - All client Multisite

OR another solution

should i make 2 different website.
a) abc.com - just to sell web hosting and
b) def.com - just to sell instant website

therefore abc.com have 1 IP and def.com have 1 IP. then i wont mind def.com IP share with all my client multisite.

I hope you understand my questions.