can i use the HPS light threw the whole grow and get the same yeild if i would of used a CFL for vegging and HPS for flowering?

Basically no, but it could be close. It depends on a lot of things. 1000 watts of flourescents probably would do better than 150 watts of HPS. It is much easier to get the results you want using high intensity lighting. If you don't want separate veg and flower chambers (twice as much weed), then you should get a switchable HID ballast and two bulbs, or one ballast, one bulb made for the ballast and one conversion bulb. For flowering, nearly everybody agrees that HPS will generally produce much higher yields than CFL for the same effort, but you can get some good bud using CFL. You just need a lot of them. For vegging, MH works best, but nearly anything will do. For germination and cloning use CFL, or keep your HID way back, becease when they are just trying to start they don't want intense light.

Is mylar very effective compared to white paint?

White paint good, mylar better, dimpled light reflector material best but not economically practical for most people.

Can i water the plants with tap water or should i use a water with the proper PH level?

Growing in dirt: Not as important. Just check it once to make sure it is not way out of the ordinary. I am not an expert here, but it should probably be between 4.5 and 8.5

Growing in Hydro: Very important. Check it every time you mix nutrients. Even more often is better. I am an expert here, and it should be 5.2, but 4.9 to 6.2 is OK. Remember the PH scale is logarythmic (sp?). That means 5.3 is 10 times more alkaline than 5.2. That seems like a lot, now I'm not sure. Or is it 6 is 10 times more than 5? You experts help me out.

Is 2 intake fans, one blowing on the plants and one blowing on the light, and one exhast fan be enough to keep a good tempature?

Depends on a lot of things. You need some kind of gentle breeze blowing on the plants no matter what. This is probably sufficient if you are using CFL. If you use HID you need a lot of ventaltion. Cool tube is best for keeping the light cool, plus you should have some other air pumped into and out of the room. HID's get hot, especially higher wattage.

Good luck.