I am not a technical security expert but here is my understanding.
It would not matter how many untrusted nodes you launched. They could only affect consensus for the quorum slices they are apart of, which do not include any trusted slices.
They could not override the trusted network.
The closest equivalent to a 51% attack would be if those nefarious nodes managed to become 66% of the trusted nodes in a trusted quorum slice. Then they could falsify transactions within that slice, but the other slices would reject those transactions.
I believe this would freeze the Stellar network until it was resolved to prevent a hardfork. Possibly only for the affected slices?