Is it silly to go to Tromso from 7 - 11 Nov? Is it too early in the season to have a chance to see the northern lights? Is it too early in the season so that it will rain mostly instead of snow? Or could we still have a really good time soaking up the atmosphere, getting on a dogsled, doing a trip with snowshoes. Worst case, we could try to postpone the trip to the beginning of December maybe...would that be better? Thanks for your advise.

Tromsø is surprisingly mild (close to the sea): both start Nov and start Dec has TYPICALLY/AVERAGE around 0 in daytime and a few deg. below at night.
Both months have LOTS of precipitation - as all winter - 100 mm/month.
So both periods are pretty equal although chances for stable dry snow on ground near coast gets higher and higher as you get into the winter. But there is probably snow om ground if you go inland.
Of course there are plenty of pitch dark nights outside the city already from September/October (all northern hemisph. have 12 hours+ from late September! - so IF weather is good AND NL is "on" the chances are good. The "snowrelated" tourist things you go for gets more chances snow and much less daylight in December. last sunrise is late November.

Alta climate: http://en.wikipedia.org/wiki/Alta,_Norway
Vadso climate: http://www.weather2travel.com/climate-guides/norway/vadso.php
A lot less precipitation than Tromso:
http://en.wikipedia.org/wiki/Tromso or http://www.weather2travel.com/climate-guides/norway/tromso.php
On this page there is a detailed graph of the last 12 months weather in Alta
http://www.yr.no/place/Norway/Finnmark/Alta/Alta/statistics.html
You can see that during last November the temperature in Alta was below freezing most of the time, and it briefly fell to below -20C. That was a bit colder than normal, but certainly in the last week of November the expected average temp is about -4C to -5C, so you can expect precipitation there to be mostly in solid form by then.

Not too early in the season - there have been some amazing displays already - last weekend saw a lot of NL activity. It's all down to luck unfortunately...

Luck, luck and more luck. While averages may give you an indication towards when to go, at the end of the day its down to having one's noble rearside located at a favorable position at the moment the Aurora Borealis does decide to show up.
Going inland to somewhere like Alta or Kautokeino will improve your chances of snow, though. Coast is usually wetter all through winter.