First. You should be using mh bulb for veg. Hps is for flowering.
If you have the light too far your plants will stretch. I'd say you could go to at least 16 inches. A lot depends on your temps in your grow room to.
I find seedlings do very well under fluorescents and that is my preferred method. After a couple of weeks of fluorescents I use HPS for the remainder. I have MH but prefer HPS. Haven't encountered any problems so far. I have a 430 watt HPS and I keep it about 12-16" above the top of the plant. I always do the back of the hand test when I adjust the light. I place my hand palm down beside the top of the plant and see if the back of my hand feels any heat. You can move your hand closer to or further from the light until you find the right distance. It's something you'll get a feel for. If it ends up too close the plants will scream at you. Hopefully you'll be listening. For seedlings I would probably raise the light to 20-24". You say you have it at 27" so that should be okay. You could lower it slowly as the seedlings grow until you find that sweet spot. HID lights penetrate well so I wouldn't be concerned about the seedlings stretching. In the wild seedlings would be shaded by other plants until they get their legs.