I use KDE only for my GUI and I use Kate for my editor. I tend to use Kate for all of my text editing, even the simplest of text files, although I'm trying to get into a habit of using KEdit for very simple files. I don't do any C programming, but I am continuing to learn more about bash programming and I like Kate's syntax highlighting for that. I also use Kate for some HTML editing.
Kate can provides tabs---sort of. There is a pre-packaged plug-in that you need to enable to provide that functionality.
I wish some more Kate plug-ins would appear, however. For example, today I wanted to sort a simple text file that I was editing in Kate. Yes, that is easily done from the command line with the sort command, but I would enjoy seeing that type of tool in Kate. I guess I got spoiled over the years with MS Word's built-in sorting.
In text only mode I dislike every editor I've tried because none use a menuing system similar to that provided in the KDE (or Windows) GUI. I tolerate vim, however, which provides syntax highlighting.
You might want to investigate Quanta Plus. I am just getting started with that environment. As already mentioned, because you are interested in C, check out KDevelop. Both tools should already be installed on your Slackware-KDE system.