Any suggestion / guidance (site addresses) on Ferries from Sweden ports (Stockholm preferred) to any German port or Amsterdam?

There are no ferries from Sweden to the Netherlands or to North Sea ports in Germany. There are no services from Stockholm to Germany. Ferries run from Göteborg or Trelleborg to Rostock, Sassnitz, Travemünde or Kiel (www.stenaline.com, www.ttline.se or www.scandlines.se).

Thanks Geoff. If we are in Stockholm what time and means we need to go to Trelleborg? Which port in Germany would you advise? Sorry for our ignorance of the area....

Which route is best depends on many factors. All the ports have rail connections and you can check train times on www.bahn.de or www.sj.se.

Geoff, thanks a lot. One question; while trying to book a hotel in Trelleborg, I'm alway thrown to "Malmö" by all sites. is this place just around the corner to Trellerborg, or what?

Look at a map, or try looking at train connections to see how close Trelleborg is to Malmö. Trelleborg is a small town with a ferry terminal, while Malmö is the third-biggest city in Sweden.
I can't see why anyone would want to stop overnight in Trelleborg - instead of staying in a hotel there, why not book a cabin on the overnight ship to Travemünde?

I agree with Geoff.

Which port in Germany you want to arrive at would largely depend on where you are headed. If Amsterdam is your next destination, Rostock should be your last choice.