put all the lights you want to use on top, if you go with your idea you probably will lose yield. all lights on top and prune all lower growth
Ok, please do not do this.
Fun fact about lower growth: if you keep it healthy and give it sufficient light, your top growth grows as it would if you pruned the bottom, AND you get a substantial yield from your bottom growth, not just a bunch of popcorn nugs. Pruning the bottom makes you lose out on 30% of your possible yield... bummer!
You're right in thinking that the plant will grow towards the light source, and you should plan for this. Are you going to circle your plants around the 1k hps and put two 400s above the plants? Given the way that a vertical setup works, this is somewhat confusing.
You're going to have plants starting at different heights, so your plants that start close to the 400 watters are are going to get more energy from the lights on top, which will positively affect the higher up plants' side and lower growth. Giving them the biggest yields. I think the reason why the hydro store guy said to have the 1k hps on top is that it offers more light penetration. However, putting the 1k hps on top might make your higher up plants grow away from your vertical set up which means their side and lower growth will get their energy from the vertical set up.
You really only need one 1k hid to give sufficient light to a vertical grow imo. One guy on here got 44 oz with 1 600 watt hid using a vertical setup... that's fucking unheard of growth! Do some research about vertical grows, it seems to be the way that most people are going nowadays for indoor.