lights, action, digitial camera

Okay, lights...

First a question:
I learned that if one had a MH ballast/setup and lamp, you could then purchase a HPS conversion lamp to use with the same ballast/setup. Question is, can you do the opposite, buy a HPS ballast/setup and later swap the lamp with a MH conversion bulb? I'm a little tight and wish to purchase the HPS lamp now and add on MH vegging later.

Ok, I'm looking at: (If you had to choose, which would it be?)
250 MH setup and (later add the HPS conversion lamp)
(question: our 'conversion' lamps the way to go? Any difference?)

8x2ft flourescents T5s veg&grow lights 16000 lumens (192watts)

My space is only 3'6X3x5.

One other concern I have, I have a sprinkler system and they ran a nozzle into the space that is now my grow. Ought I be concerned? I put something around it to prevent bumping it. It theory the little wedge of glass needs to melt before it kicks off the system. So I can look at it has a security feature. Just concerned that Murphy might decide to visit me again and who knows what...

Thanks for taking the time to read and again for any input!

In the perfect setup you would use both hps and a mh at the same time, Mh is best for veg and hps for flower. I use both from start to finish and it makes a big difference. I know alot of setups are closet and don't have the room.
You can get a 1000 watt hps ballast and the solarmax1 bulb which has both 400mh & 600hps in the same bulb. If vented properly this could be in a closet.

I thought you ran with MH then switch to HPS, you're saying, both at the same time?

Thanks for the input.