If you have to choose between a 250 watt conversion bulb, HPS + MH or a 400 watt hps,

go with the 400watt hps

if you are going to grow 10 plants you need to keep them short.
a 400 watt bulb does not have much penetration and with so many plants in a small area, it will further reduce your penetration.

You are best flowering your plants at about 12 inches. You can grow them to 12 inches.. clone them and then let them get 12 inches again..

are you starting from seed?

get a fan and have it blow between your plants and the light... and keep that 400 watt bulb as close as you can without burning your plants...

no farther than 10 inches during flowering..... less than 6 inches is ideal..

this will give you greater penetration and THICKER HEAVIER buds..

1000 watt is "better".. but electricty costs in the USA have sky rocketed..

it is ok to start with a 400

LOVE
KNOWM