How efficient is iron smelting ?
If they want to carve/excavate stone, they're going to want tools to do this - even if they're just picks and shovels. Though I'd imagine they'd prefer tunnel boring machines, so they can dig out large areas quickly for extra habitat space. I'd imagine iron has quite a lot of other uses besides building and stone carving too. Also, stone carving is a specialised skill just like iron working is. Even more so today, when stone isn't so common as a building mateiral.
The need for specialists may well turn out to be a problem. I say 50 people, but it's possible with 650 tons to spare more could be supported (I am currently refining my models - I'm almost certain the number can go up quite a lot). A lot of automation might be required. I'll have to look into it.