I agree that the much more likely option is having them all be white, or 7 pale Inklings and 1 sort of tanned one; but I just wanted to point out how neatly Splatoon's existing options fit into the way Smash 4 categorises alts, should they want to use all the options available to them. After all, the Villager shows as much variance as was possible in the game they were basing them off; and giving the Inklings all of their possible skintones would take far less work than that.
For the outfits, who can say. There's so much variety in Splatoon that replicating the ones from the marketing, creating entirely new ones, or just going with different shirt designs and the Agent 3 armour are all equally plausible. The precedent of Villager would suggest they'd go for new models for each, while Robin shows a single model per gender with different palettes is also a possibility.