It has been many years since I had a wood boat so it is taking awhile to jog my memory so some advice may come randomly. I don't know how lumber is processed in Turkey
but here much of what is used in the construction industry is kiln dried (using heat to speed up the drying process) and is to dry to be used as planking. Much of this wood is poor quality so it is a moot point anyway with the exception of western red cedar which is readily available green with a high moisture content. This cedar is an excellent planking lumber and certain types of pine are also commonly used. There are differant types of pine and cedar and not all are acceptable and they also differ in differant parts
of the world so I can't advise on what's best for you to use. Using the same as what the current
planking is probably best. Most boat builders either mill their own stock or work with a mill. This wood is cut green and air dried out of direct sunlight to a certain moisture content and then it is ready to use. Kiln drying makes the wood to dry and it will never be acceptable as planking stock. It would seem to make sense to me to use lumber that is not to dry for the replacement planks and make them fit tight. Hopefully the rest of the planking will swell back to it's original shape and the new will swell just enough to tighten up the new tighter fitting seams.
The caulking bevel is only on one edge of the plank and this edge fits against the straight edge on it's neighboring plank. This bevel changes along the plank lenght depending on the curve of the frames and twist in the plank. With flat runs of planking this change may be little to none. Determining bevels is to much to explain here so a good book with diagrams would be the best way to learn the process.
I think that would better if you lenghtened that one plank. One thing to think about is how much is left of the original plank after you have cut off the bad area? If after cutting off the bad area and a majority of the plank is gone it may be best to replace the whole plank rather than have a bunch of short planks. If the planks are fairly long then it is to much work to replace them entirely.