I am using a 600 W HPS for veg, is that ok because the suggetion is for MH

It will work just fine. HPS wont be as good as MH but your plants will still grow huge. I've seen it done hundreds of times and I do ti myself.

hps is fine for veg imo

400whps all fine no huge spaced plants..

The only potential downside is they might stretch a little more than if you had used the proper spectrum of light, but in general it's really minor and not worth worrying about. More of an issue if you are growing in a little cabinet that just doesn't have the height or with a smaller light that can't support tall plants.

Eventually you should change to using a seperate area to veg with a smaller wattage light though. 600W is probably four times the light you need in veg and you could save a lot of money using less light. Or better yet if you veg in another area then you can flip grows really fast. You could have clones always ready to flower with just 200w or less, some basic 4' flouro fixtures is usually enough.

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It also helps if you have an air cooled reflector so you can put the light closer to the plants to prevent stretching.