I think the biggest flaw in your scenario is the assumption that you get a worthwhile yield from paying to send your companions on gathering missions. Assuming the numbers work out anything like they were at the end of beta, you don't.
You're basically looking at a return of 2-6 units (assuming the mission doesn't fail outright) and a cost that is significant (starting ~400 credits per run at tier 1). Since you appear to be looking to gather resources to sell, I'll be blunt: Sending your companions on gathering missions is going to be the least effective way to make money. Anyone actually gathering resources from the field will be able to undercut you tremendously.
Taking three gathering skills is a money maker because it lets you gather almost every node you come across. Gathering missions rarely yield returns that match the time/credits invested.
Even slicing is subject to this - ignoring failures for the moment, about 2/5 slicing missions will result in a loss. Another 2/5 will result in a slight gain. The last fifth will result in a significant gain, often sufficient to offset losses. The net effect is a gradual increase in credits, but like other gathering skills, where slicing really shines is in the field. (Run around questing, get a pile of bonus credits every few minutes.)
If you want to make money from sending your companions on missions, you want mission skills, not gathering skills. Diplomacy, underworld trading, investigation, treasure hunting. These yield rewards that can only be gained through missions, thus you're not competing with people who can obtain the same items for "free" while questing.
Likewise, this is where your idea applies - you could send five companions on underworld trading missions, or two each on diplomacy, underworld trading, plus one on investigation. Focusing on one mission skill will probably be more effective, as you essentially have more chances to retrieve higher quality rewards (schematics, etc).
Thanks. That was a lot of good info.
Actually, my question was not so much about making money on sales but to have my alts help each other most effectively. I know I'll end up with four characters, so I was thinking about the best way to distribute the skills overall. It sounds like it's worth it to have one character who is a grind gatherer without slicing. That would leave the other 3 with having a craft, slicing and a mission skill.