I think a good rule of thumb is to say “in another RPG, would this thing get its own character sheet?”. If the answer is YES, then it deserves to have its own cliches in Risus.
For example, in most space RPGs the spaceship gets its own character sheet. I haven’t played a lot of “Mecha” games, but it seems to me that in those games the Mecha gets its own sheet. Also in Car Wars each car had its own character sheet. If you played Risus in these types of settings, it would be then a good idea to give these types of vehicles their own character sheet with their own cliches.
Also in Ars Magica the covenant (essentially, the complex where the characters lived) would get its own character sheet (more or less). This I think would also get its own cliches in Risus.
But for example, when you play D&D, your horse doesn’t get its own character sheet. So in Risus, it is just a Tool of the Trade. But maybe in some games a horse would get its own character sheet. It depends on how important horses are to the game world. In Risus, you don’t have to always use the same game mechanic for the same situation. In one game, you can decide that something needs to be very detailed, and so use combat, multiple cliches, and so on, but in another game, even with a similar genre, you can decide that the very same thing doesn’t need to be so detailed, and so perhaps you use single action conflict and tools of the trade.