I'm not sure from your post if you get the process, so please forgive me if you know all of this.
Some DSO images, especially when shooting narrow band, may require quite long duration exposures on the IMAGING camera. The guide camera shoots a much shorter exposure as it repeatedly takes measurements of a bright star for the purpose of correcting the mount's tracking in order to minimize tracking error while the IMAGING camera shoots its long exposure.
From a practical standpoint: Many people have used the ASI-120 family as guide cameras. In fact, one version of the camera is sold for exactly that purpose, but it is a relatively low-end camera in terms of sensitivity and works better in some situations (eg: a properly sampled separate guide scope) than in others (eg: long focal length OAG). Moreover, while some people do successfully guide with OSC cameras, they are inheretly less sensitive and also suffer from sensitivity differences between adjacent pixels which make them less desirable for calculating a centroid and guiding than a monochrome camera.
Since you already have the camera, by all means try it. It may work just fine. If it doesn't work then you can cross that bridge when you come to it by buying something like an ASI-290 mm Mini or an ASI-174 mm Mini depending on the resolution of your guiding optics and FOV needs*.
* = Don't get sucked into the "ratio myth" that says that your guide camera's resolution should be related to that of your imaging camera. Due to sub-pixel centroid calculation algorithms, a well-sampled guide camera should be able to guide your mount as best as it can be guided, regardless of the imaging train's imaging scale.