I know there are a ton of these, but it seems everyone has their own requirements. I would be grateful if you could advise.

I plan to be sleeping in temperatures around freezing (30F) with just a tarp. I currently have a LaFuma 600 down rated for 40F with a ThermoMax sleeping bag liner. I'm worried about two things: 1) it won't be warm enough and 2) fog and dew in the mornings. I don't expect to sleep outside in heavy rain.

I want something warmer, but still with a small stuff size. The LaFuma is 6x12".

I have a couple questions,
1) It seems I want down for the small size, but is it a really bad idea if there will be heavy dew in the mornings?
2) Do most sleeping bag zippers mate with each other? (It would be a huge plus if I could get a warmer bag, but still be able to join it with my current one.)
3) Should I instead spend my money on a Thermarest sleeping pad? (I currently have a $5 foam pad.)
4) Any sleeping bag recommendations? I'm also in France, if you can recommend a good online/B&M shop.

Thanks so much!