I have seen both Foundry Darkest Africa, specifically Zanzibaris and tribal guys, and Swashbuckler/Pirate figures on the same table and didn't notice any size difference problem. There are some size differences between the some of the early pirate line and later figures, but well within the range of human variation. Obviously don't want a regimented look when dealing with individually mounted figures for something as wild as pirates.
Slave soldiers? Whose slaves? I don't know much about Sudan, but for Congo I would guess that Foundry tribal musketeers and Ruga Ruga might fit the bill. Maybe generic askari could be used as slaves of Zanzibari types, but not sure.
Wouldn't want to use the tribal musketeers as askaris. The Foundry tribal musketeers are wearing almost nothing, and European's seem to have used askaris wearing more clothing.