Well. Strictly speaking a maxed Pink pet can't even get lv10 normal skills. So I doubt a full formation of maxed Pink pets will mean lv10 soul skills. Maybe Lv6-7 or so? I'm thinking there would eventually be Gold pets, and to that end I'm sure a full formation of Gold pets would give max soul skills.
I can't speak for the exact formula for leveling soul skills though. It would require more pets of varying levels than I have access to in order to test and figure out. I do know that using multiples of the same Type in formation is pointless, though. So if a soul skill requires a Lv50 Elemental type, using multiple Elemental types will not add more bonus levels to this soul skill than if you used 1 elemental and any other type(s) with it. The requirement should really be looked at as a requirement, and not being relevant beyond that.
Regardless of what the EXACT formula is doesn't especially matter. Just get all of your pets up as high as possible since it's clearly related to your pet formation's total levels somehow. And keep in mind, soul skills with a higher level requirement will level slightly more slowly than those with lower requirements. So if one skill requires a Lv30 Beast, and another requires a Lv50 Elemental, if you meet these requirements and fill out your formation you'll probably notice the Beast skill is Lv3 while the Elemental will be Lv2.