Delurking again...
They'd not be new converts right after meeting the cell, so to speak. They'd be used whenever they could cause the most damage. And they'll steal the material and facilities from Starfleet. The expertise comes with a Singer.
One more point I'd like to make:
The Harmony are capital P paranoid about where they make their implants and who has access to even the finished implants, let alone design and production hardware. Note that despite the fact that they literally owned the Tauni, and then were forced out, the Tauni rebellion found no production sites for anything in their territory.
The idea that they are suddenly going to give the detailed schematics for it's production to covert operation agents to establish production sites in hostile territory of a peer power is lunacy. Consider the security levels of designs and production of capital ships are secured as a minimum starting point for how the Harmony secures this stuff.
This is the centerpiece of the Singer network.
An enemy who got hold of the designs could reverse-engineer the hardware and either disseminate an immunizing infection through Harmony space and it's citizens, or actively hacking the network and either going after the Singers themselves, or broadcasting conflicting instructions to Harmony citizens. Or just listen in, and gather intelligence.
And that's just off the top of my head.They are NOT cavalier about disseminating this stuff.
At worst, those big mother ships of theirs might have small production facilities. Everything else is buried deep in the Harmony and shipped from there.
EDIT
There are also a limited number of Singers, and even fewer who have covert ops experience. Look at the Singer interlude we just saw. Does whatsherface playing Barbie dolls look like she has the aptitude or patience to run a covert op in Federation space?
And all this assumes they even have the bandwith to control multiple people in Federation space. Instead of simply leave instructions and move on.
Seriously, don't overhype the Singer threat. They have significant limitations of operation outside HoH space without a Sanctuary in range.