That's what the locals are for. The naval base will bring some settlers, but by far the majority of the construction personnel will go home, and from then on the majority of Europeans on site will be military personnel who normally reside in Europe.
And I'm not sure how much more it would have gotten after that, it's competing with the rest of Ethiopia after 1936.
Libya had a smaller native population than Italian Somaliland, and a far far smaller native population than the Somalia Governorate.
Which is why the population increased by 40k in ~ 10 years, and didn't really show much in a way of slowing growth until WW2? Somalia and Eritrea were safer and had a much larger Italian population than further in Ethiopia, even if the climate is better in the highlands.
And, once Ethiopia eventually is lost (they'll not be keeping that), the population that settled in Ethiopia will flow to
And the Italians didn't have to settle and could have used natives to run the burgeoning mills and factories that were being set up in Mogadishu and elsewhere, but settlement was encouraged and the Italians worked in those mills as well - it wasn't purely native population. Italian settlement wasn't widespread and such outside of the central triangle OTL, so you have a possible Oran situation, long term.
To construct a naval base, I'm not talking about the labor to build it. The labor to build it doesn't matter. It's the engineers and trades that actually work in the shipyard, which was to become the biggest naval base in East Africa (like Eritrea was designated the industrial center). To run a fully functional naval base, you need thousands of personnel trained, and almost all of those are going to be imports. If it's a small naval base, you're looking at about 10 thousand people, very roughly - and this was not to be a small naval base. That's not counting military personnel, of course, that's just what would be necessary to run the base. Let's not even include those personnel that would bring families along (or those that would marry native Somali or others), or otherwise.
So, that population wouldn't be a transient one that lives overseas - it'd have to be at least semi-permanent, which means settlement. And the only point that is that it'd be another core of settlement for Italians to migrate to. At first it's just the base, and then the base workers, and then the base continues to expand with immigration from Italy, natural growth, and sailors and similar deciding to remain (not many, likely, but they'd add in). So you have a second population center outside of the Mogadishu triangle for the Italians.
Also, Libya's population was ~860k in 1940, and Italian Somalia ~1150k in 1939, which is the closest comparison I have at this point in time. Libya is smaller, but not necessarily drastically smaller. (Eritrea seems to have been even smaller, too, than Libya).
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The key point is that Somalia is, until oil is discovered in Libya, more valuable in a strategic sense. It's the farthest flung colony, with a relatively quiet native population, with good agricultural potential, and allows the Italians to project power. Libya isn't nearly as valuable. So, the fascist government certainly would have continued to encourage Italians to settle in Somalia, which it had been quite successful in doing at that point in time. Natural growth, continued immigration due to strategic importance in securing the region and projecting power, and other efforts would continue until the fall of the regime (not out of the question, of course), or the discovery of Libyan oil, at which point Libya becomes a much more important colony and the immigration from Italy will dry to a trickle, at least for a while.