I still think that it would be more fun if the summoner of Hyborem or Basium got a vassalage agreement with the new civ rather than nothing/a permanent alliance. The human should probably be the master whether (s)he switches or not, but that does leave the question as to what to do when an AI reaches Infernal Pact first.
The game should definitely be made to increase the size of the starting armies based on the strength of other civs and/or the prevalence of the appropriate religions.
I think that Hyborem and Basium need to start with some late game techs, or possibly techs that no one else can even research, in addition to their summoner's techs. Thematically, archangels should have a deeper knowledge of the world than mere mortals. Mechanically, they need a technological advantage early, since research in their tiny new empires really fall behind their rivals. Also, if you put a large weight on this tech, it can artificially inflate their scores and so make them play more aggressively.