Ballasts and Wattages !!!!

Ive currently got a lumi 600-400 watt digital ballast (with lumen booster) its useable for MH and HPS bulbs and ive been running a 600 watt MH bulb.

at the minute im down to only 2 plants, can i put a 250 watt HPS bulb in, Im assuming i can as it will only try draw lower, as opposed to putting a 1KW bulb in and drawing to much and cooking the ballast ...

Is my thinking behind this correct ? as 400watts seems a waste for two plants !

No to a 250w bulb in 400w ballast.

the arc tube in a 250 watt bulb will not last long when fed from a 400 watt ballast

400 watts is never a waste on 2 plants, more light = more buds. i hope your switching out your mh light for hps to flower?
get a 600 super lumen bulb and whack that ballast up.