Hi RalfG. I think a lot depends on what you plan to do/see on each city and what kind of traveler you are. When exactly on spring you plan to visit too.
I am not an expert in Poland but I spent about 25 nights on January/February 2011 there on a solo trip, visiting Warsaw,Torun,Malbork,Gdansk,Poznan,Wroclaw,Krakow . This trip felt fast paced to me but I tried to visit some friends on various locations. Usually I travel much more slowly. Most of the time I enjoyed the country by my self and I just met with friends for a meal or a few drinks at some cities..Usually I like to take it very easy, I am very interested to explore as many sites and museums as possible without rushing, and also to walk around various areas of each city, visit parks, town squares, to have a long meal and people watch etc... If you are this type of traveler I think that you do not really want to add more destinations on your list, you may even feel rushed as it is now. Actually I am not sure if the 1 night in Torun would be better spend on Wroclaw, as 2 nights/1 full day there is definitely not enough, while 1 overnight in Torun is ok to have a look around the town, but not enough to visit many sites either... I visit Wroclaw only for 2 nights my self but I think this area deserves at least 4-5 nights, very scenic city with a great vibe and lots of nice sites...I wanted to have a quick look on Torun and break my journey from Warsaw to Gdansk with one Torun and one Malbork overnight, although this is not my style of traveling at all ... I think that 1 night in Malbork about made it as more or less main interest on Malbork is the castle, but this was not a wise decision for Torun if you really like to visit some sites, a few nice museums etc there... Now about the usual Warsaw/Krakow dilemma...I find that people tend to under-rate Warsaw and give more credit to Krakow,but I think this is a bit unfair... I spent 5 nights at start of my trip and 2 nights at the end in Warsaw, and I really liked it. There are some nice sites in Warsaw suburbs and various sites spread out around the city while most sites in Krakow are concentrated in one spot, plus Krakow was not so influenced by WW2 bombards so Krakow seems a bit more attractive...but I really think each city deserves a good 5 nights/4 full days at least, ideally 7 nights if possible. I spent 5 nights in Krakow taking one side trip to Wieliszka Salt Mines and one side trip trip to Auschwitz-Birkenau and I think 5 nights were definitely not enough for me on the area.If you plan to stick strictly in the city its self, then I would think maybe 5 nights are about right... About Gdansk, I pleasantly spent 5 nights/4 days in Gdansk enjoying just the historical center without straying to the coast areas,so if you are interested on those, you should consider more days. I liked Poznan and I managed to visit a few sites, but this is the only destination where a friend came to join me from an other town. I spend 2 nights plus most of next day with her and 2 nights by my self and so we spent a lot of time to catch up on each others news and I did not get to visit too many museums, sites etc. I think 3-4 nights are necessary to visit most of those and have a few nice walks and enjoy some of its vibrant nightlife though...
Note that I visited on a time of year where dark was coming early and weather conditions were something to take into consideration. You may fit more in a day latter in season, but allow for some spare time to relax or take in something you haven't consider, or just in case of trains cancellations or delays or whatever...
I connected among those places in trains, most of trains were rather on the old and slow side, but nothing extremely inconvenient. I think public transport might have improved since my visit due to the Euro 2012 anyway...