My understanding is that while the AI always tries to keep its Science-slider at max. (i.e. 100% in unmodded C3C) and won't adjust its Lux-slider, it is still bound by the slider-limits.
So if you want to level the playing field (/ limit the human advantage) at least a little bit, one possibility might be to lower the slider "rate-caps" for each of the governments in your mod. As I understand it, these caps apply to both the Science and Lux-sliders, and to all Civs in the game (including the human player). What I'm not sure about is what the minimum rate-cap is, nor whether the "virtual Tax-slider" is also subject to the cap. That is, if the minimum cap were e.g. 40%, and the Tax-slider is also affected by the cap, then after paying for Science and Tax at 40% each, the AI would essentially be "forced" to convert the remaining 20% of its commerce into Lux-Happies. But if the minimum rate-cap is 50%, then the AI will still not use the Lux-slider (but should at least have more money available for rushes/ upgrades/ trade).
Regarding Specialists, the AI will also use the Scientist and/or Taxman specialists (depending on Slider-settings), but AFAICT, only in addition to Clowns, after the town has been made content, and the town has insufficient tiles available for its population to work but still has excess food to support Specialists (given the AI's tendency to plant at CxxxxC, this usually doesn't happen before Metropolis-size). But if you had Clowns give 2 Happyfaces instead of only 1 Happyface, then the AI wouldn't 'need' to assign so many of them to maintain order, so might then be more likely to use the other Specialists.
There has to be at least one Specialist available from the start, to allow (manual) tile-reassignments, but is there any reason why that should be the Clown rather than one of the others? If not, why not make the Geek the default option (i.e. not requiring a tech), and have the Clown unlocked with Literature (or even FreeArtistry), and the Taxman unlocked with Currency (or even Banking)? That way, you could maybe force the AI to choose the "better" Specialist(s) during the early game, when it makes the most difference (and as the human would do).
Conversely, (AFAIK) the AI won't use Policemen or Civil Engineers at all, so a different approach is needed. Since these Specialists are essentially "human-use-only", I have made them require "payment" (i.e. they 'add' –1 GPT to the town; can be done even in the FxsEditor), so that there is a small per-capita economic penalty associated with using them.