I have some rapidograph technical pens and want something waterproof in them.
Airbrush acrylics are not going to be necessarily waterproof, and it can take several days for the retarder in them to evaporate before they become more water-resistant. However, the pigments are lightfast, and much better for long-term than inks. Spraying Airbrush Transparent Extender over them can fix them after they dry.

What are my options in the event there is some drying in the tip of a pen?
Airbrush Colors have a good deal of retarder in them to prevent drying in an airbrush, but if you need to increase the open time further, and therefore decrease the chance of clogging, add Airbrush Medium. However, do not leave the paint in for too long of a time, as the pigment begins to settle within a day or so. If the amount of open time is fine, but you want to make the color a little less intense, add Airbrush Transparent Extender.

What is the smallest size tip of a technical pen I can attempt with Airbrush Acrylics?
The Airbrush Colors are meant to be used in the finest of airbrushes, and the smallest nozzle size we have tested is .18 mm. Looking at the various Rapidiograph, Staedtler and Rotring pens, We'd have to say the 0 (.35mm) size is about the smallest most colors will work well in, but some of the larger pigmented ones like Naphthol Red Light would start skipping rather quickly. In general, a 2 (.60 mm) should work well with all of the airbrush colors, but just like with an airbrush, much will depend on how much use it has to undergo at one application.