Slow and Purposeful is worded with - "A unit that contains at least one model..."
- So one slow and purposeful model in a unit means the whole unit has slow and purposeful.
Relentless, on the other hand, is worded - "Relentless models can..."
- So only a model with the relentless rule is relentless, his unit is not (unless they also have the relentless USR)
The key is in the wording.
Any rule that includes the phrase "a unit that contains at least one model..." means the whole unit has it of one model in the unit has it.
Any rule that does not include that line only applies to the individual models that possess that special rule.