i was planning on bringing layers of clothing...thermals..wool jumper and a fleece. But im reading that people recommend bringing a jacket. What kind?? We want to travel light and rent a sleeping bag for the salt flat tours...Do we really need a jacket?

On the salt flat tour I and everyone in our vehicle wore down jackets... plus as many layers of clothes as we could fit. We all looked like the Michellin man.

You will definitely need something to resist the wind. If you have many layers you could maybe get away with a rain jacket or light windbreaker. I've just come back from there and wore an icebreaker woollen thermal, cotton long sleeved top, cotton shirt, cardigan and a cheap down jacket, which I bought locally. It is cold at night and the wind is vicious, so make sure your hat covers your ears. If you want to pack light there are loads of markets you can buy cheap clothes in La Paz and you can just donate the clothing to your guides or assistants when you are done.

On the salar the only difference between summer and winter is wet and dry. You show up at Lagune Colorado...the coldest stop...at night without a coat you better be able to kill someone and take theirs. You will certainly be willing to do so.
You might be Ok with many layers and thermals but you will definitely need something to keep the wind out. If you do an overnight stop at the hostels at Lagune Colorado then try to get a bed away from the outside walls, By chance, I got a bed in the middle of the room and had a good nights sleep with my hired sleeping bag, my wife got a bed near the outside wall and woke up with icicles. Also make sure you have sunglasses during the day.
Well, I never travel as light as I would like because I have a lot of photography equipment, but my friend who was with me in Bolivia had the same stuff I did--lots of warm layers plus an outer jacket, all in the exact same 62L backpack I had. (We also rented sleeping bags). Do you want to travel even lighter than this? We went in mid-September and it was damn cold.

We had one small daypack between two of us. We hired sleeping bags and took the following.
2 silk sleeping bag liners.
hats
gloves
scarves
4 pairs socks
2 pairs trousers
underwear
sleep clothes
boots
thermal long johns and t shirts
cardigan or jumper each
coat
food, crackers, chocolate, nuts
flashlights with spare batteries
toilet paper
soap
camera.
sunglasses
sunblock
wet wipes, very important!!
bandanas, also for use as handtowels. You won't be showering, probably, it's too cold!!
ipod, if the car has a jack, good for the long journeys.
money, for tips, snacks, entrance fees, beer.
That was it. You're wearing most of your clothes most of the time. You really don't need to bring much stuff. It's just something else to worry about and you can leave the bulk of your stuff in the tour operators office.