Ru'afo claimed the Federation was old and weak since it was always fending off attacks from the likes of the Klingons, Borg and now the Dominion. Plus he killed Dougherty and ordered an attack on the Enterprise, I don't think they had any great fear of Starfleet.
Alright DWF, then why did the Sona form a partnership with the Federation?
The Sona knew the location of the planet.
The Sona knew of the medical properties of the particles
The Sona had the technology to do the harvest.
In term of just
physically acquiring the particles, the Federation brings nothing to the game. So why did the Sona want them in it? My read is that the Sona were worried about long range retribution from the Federation, so they approached the Federation Council and made them partners.
However, if the Sona had no real fear of the Federation, why include them at all?