Colonize them somehow?
The only way that you can 'colonize' a world controlled by a full-fledged empire is to conquer it. At least, unless you can somehow or other get the world to become independent and non-hostile.
If they're getting in your way, taking them out isn't a bad idea, particularly since you're playing Sluken. One of their racial victory conditions has something to do with enslaving or exterminating non-Sluken, if I recall correctly, and I think there's another one having to do with fighting wars. This isn't really a consideration if you're not playing with racial victory conditions on, though. Plus, I think the Sluken are supposed to get a growth bonus on colonies where you're exterminating people. Otherwise, if they're really that much weaker than you are, especially if they don't yet have hyperdrives, you can always just expand around them and more or less ignore the fact that they exist. I'd make certain those independents don't stay independent long enough for the full empire to incorporate them somehow, though.
Homeworld assaults can require a fairly significant invasion force, particularly if they have a good spaceport and several defense bases up to defend the planet, so remember to bring a big fleet to clear out the defenses and a big army to crush the defenders, if you decide to conquer the planet. Humans are probably on a continental, while Sluken like Marshy Swamps, so the defenders are going to have a +10% bonus from the planet type and you might have a -10% for the same, though if you secure space superiority first, that isn't so much of a problem. They'll also probably have enough infantry to gain the +25% Fortified Line bonus. If you have armored divisions, you can counter that with an armoured breakthrough for +25% on your end; if not, you'll have to make up the difference in troop numbers or quality. Remember that your newly-recruited Sluken infantry units are only about 8% better than newly-recruited Human infantry units if they don't have Emeros crystal stockpiled on the homeworld, and only 3% better if they do, so your troops are more or less equal on a 1 to 1 basis. Thus, if you decide to invade, you'll probably want to outnumber them by at least 3 to 2, and probably more like 3 to 1 to account for the planetary population and defending bonuses.