I don't see your argument- you say 4% of the colony would be required for child-rearing duties (so, 40 people, which seems a bit high, but whatever) and posit that as a fundamental barrier? There's still 960 (adult) colonists left over, and it's hard to see how 960 is a less tenable number than 1000.

More generally- sure, there are well over a thousand specialist jobs that make our Earth-bound economy go. Probably more like millions. But that economy builds way more than is required for self-sustainability. The Martian lifestyle is necessarily going to be much more spare; so there's your "labor cuts."