This should be no problem to do. But you should keep in mind that propper planning is needed.
I operate an commercial RIB safari / Searafting company in Trondheim. I can give you some advices an what to wear, water and weather info etc.
I only know where to get fuel between Krisiansund and Namsos, but it easy to get more info on this from different places on net, but many of those pages are in norwegian.
Navigation is not a problem at all in the fjords etc. Just use what you learned. There are a few local 5 knots zones, but they are poorly marked. Keep in mind that not every dangerous rock has an mark on it, there are millions of them some places.
In general all harbour are 5 knot zones.
Regarding weather. I t can change from calm to gale in moments. Be prepared to navigate your RIB in bad weather. But mostley the weather is calm.
at my website I have a few pictures from Trondelag: www.fjordrafting.no