If you're wanting to allow bikers, utes and infantry to wield xbows, small arms, and other weapons without having to make a separate unit type for each possible combination, you could try having each weapon as a fixed "promotion" that can be assigned to any unit. FfH (I know, stop mentioning it..

) uses this to allow many unit types to carry Bronze, Iron, or Mithril weapons; its probably possible to borrow this code.
I'm not sure, but it might also be possible to cap the number of units that carry a certain weapon type based on how many resources you control. Alternatively, units carrying Crossbows could have an advantage in some situations (defending cities?) or there could be a rock-paper-scissors mechanism of relative advantages to discourage overreliance on a single unit type.