You can fill up wirh caltex stuff after a full flush out (use a garden hose), if you are experiencing rust in the system then your coolant mix is probably dead or extremely watered down.
A very light acid flush is a very good idea before changing entire coolant charge, specially on a diesel engine, I personally would prefer to use CAT Cummins or Shell ELC but havoline/caltex also works ok. Make sure you know how to get it to 50% concentration after flushing out.
To do that you have to find out entire coolant capacity and then measure how much drains from a simple drain of the radiator. The difference is what can tell you how to get a true 50% mix in the system.
E.g. a 10 litre system which drains only 6 litres means after a water flush you need to add 5 litres concentrate to the system and then add distilled/RO water.
For true measure you need to also measure the volume of coolant in the expansion tank, its volume is also included the total,
Meaning if your tank holds 1 litre and total is 10 litres, that means a total of 9 litres is engine/radiator. Meaning instead of 5 you now only add 4.5 litres of concentrate and fill rest with water.
The expansion tank is always filled with 50% mixture and always a bit extra to accomodate for air pockets in system which will be fulfilled after shutdown and cool-off with the extra coolant mix you added.
Hope it helps, good luck!