Yes, I think the best thing is to build a kit and see how it goes. Many people decide after their first kit that they really like doing this stuff and go on to delve deeper into the design and theory.
Ian's stuff is open source, so you can check it out as much as you want before making any decisions. Manuals, schematics, code, etc. are all here:
www.nixieclock.biz/Manuals.html
Scroll down a ways to find Classic Rev. 6.
To be clear: I'm steering you toward the Classic because that board does not care what the pin arrangement of your tubes is (Modular and All-In-One are for specific tubes). and Rev. 6 because of the spare outputs that are available only on that version. Classic works with all tubes designed for ~170V (that would be most every Nixie made, including GN-4) as long as they are not biquinary (GN-4 is not).
I don't have a specific plan for the Dekatron. It will take some creativity but it can be done. Dekatrons are not my forte but I do know there are Dekatron "spinner" circuits and even some kits online. My general plan would be to use a spare output on the clock to trigger the Dekatron spinner to spin once per second, or perhaps pulse it 10 times per second to "step" it. The clock code would require additions to control the output and the spinner circuit might need some mods too. It shouldn't be super complicated, but it's not something I can bang out off the top of my head in a forum post either. Do be aware that you need to use a clock kit that is open source so you can modify the code.