I would say, that it should work with alternate forms. But if there is an error in the code then you can check the method "pbChooseEncounter(x,y,repel=false)", more precisely the part beginning with "if USERESCUECHAIN ==true".
One thing, it is not important which form you chain, Rattata and Alolan Rattata count for the same pokemon family. And on the other hand, if you chained long enough, then, I would say, Raticate and Alolan Raticate can spawn. Mayby an alternate form can only occure in certain situations, for example depending on the trainer-ID or the map-region. Then chaining a pokemon will only spawn evolved pokemon of allowed forms. Please make sure, that Alolan Raticate does not have such a restriction.