My two cents- either way I wouldn't suggest starting in the buckets or starting with them in cups under a 600w especially a hps the reasons being starting in a buckets fine it just seems silly but I wouldn't use solo cups either, i prefer peat pots personally both work fine though. As for the light I wouldn't bother pulling that much power for a seedling for a few reasons . Those first leaves up until they start getting three fingered leaves don't have much surface area so a lot of that power's just hitting soil also I find when they're that small watering is more of a pain in the ass the ass then it needs to be if they're under that bright of a light because the soil is completely unshaded and dry's quickly (tried this same thing before), the roots at this time are little baby hairs and can end up in just dry soil, alternatively you may see it's dry on top, not want to disturb the soil checking below the top layer and end up overwatering, the last thing is the fact that it's hps, the plants may stretch and tip over, they can be proped up and whatnot by why do extra dicking. In my opinion I'd start them in small, 4 inch across or so, peat pots, put them under the t5 and let em do their thing just keep in mind to remove the pot, after you water just pull off the bottom when I'ts wet and tears easy, no need to make the roots work harder to get through the bottom of the pot when you can take it off especially with the predetermined life span of an auto, I use peat because the transplant is easier and I think less stressful. All personal opinion, the way you were thinking will work I just dont think the extra work and prosible stretching is worth it.
thank you for the advice everyone it was much appreciated