You can’t determine scaling potential from TDS readings. You need to know alkalinity and hardness (and what the alkalinity is from - usually bicarbonates). With low readings, and especially when you know the water has been through RO, you can say it’s pretty certainly not going to scale.
With regards to the service boiler, I assume this is after some use. Given the mineral content has increased with you will need to know alkalinity and hardness.