Vivi's maximum RCV is 477 at level 99, not including cotton candy. She recovers 5X that amount, or 2385. The way you make her capable for zombie teams is just to make sure your entire team has no more than double (or if you're using evolved Pell, triple) total HP that Vivi can recover per turn. You don't need slots to make this work.
Autoheal slots work independently from Vivi's Captain Ability and RCV. They're not really related at all. If a team has a total of 5 Autoheal slots, that just means you get 100 additional HP heal per turn, regardless of whether you have Vivi or Enel. At level 5, which would be 24 slots total on your team, you'd get 1000 HP per turn.
The reason why zombie teams aren't used for farming is because it takes way too long to tank through a raid boss. People just use them to guarantee one copy at low levels. With Vivi/Laboon as captains, your damage will be pathetic, so you're really just staying alive while you whittle down a raid boss. This can take over 30 minutes to an hour, depending on your team. They also don't work for any raids other than Mihawk, Monster Chopper, and Garp on global.
The 7X healer you're talking about is Valentine Vivi. The difference is pretty negligible in the context of zombie teams, since zombie teams will always be inherently weak. If you're farming Mihawk reliably and have Legends, you really don't need to mess around with zombie teams anymore. They're basically a last resort for players to pick up single copies of raid bosses.