Would those manage to spread beyond the Malagasy community though? I have a hard time imagining that the general population would want to take up what to them would be a religion of slaves.
There's several imponderables. The first one is whether the Malagasy would preserve much (if any) of their own religion, even amongst themselves. That's likely, of course, but not certain, depending on the whole range of other factors I mentioned before (especially if there turns out to be a new state religion).
The second question is how rigid the slavery lines are in Teegal. Would slavery be a permanent condition, passed down through the generations? It depends on several things too, particularly whether the Dutch get to dictate things. To the Atjuntja, slavery normally has been for life up until now, simply because it was viewed as punishment for a crime. But they have the cultural equivalent of forced labour for part of the year, and the question is whether they will view Malagasy slaves as being part of that system too. (Especially those born into slavery). A few slaves at least may well be freed, if only because they've pleased their owners, and those former slaves may in turn establish a free Malagasy community. Which leads in turn to...
The third question is how rigid the ethnic lines are in Teegal. Will slavery become strictly racially based? That would be one method, but there are complications. Atjuntja men care about their sons (more than their daughters, it must be said) and don't care so much who the mother is. So the idea of slavery being defined by the mother's status (as happened in race-based slavery elsewhere) probably won't work, since if an Atjuntja man has a son by a slave wife/mistress, he will acknowledge the son and want to raise him. That may lead to some intermingling and mixing of the Malagasy and broader communities.
Plus, of course, there's the simple demographic fact that mixed-heritage children (Atjuntja-Malagasy or Atjuntja-Dutch) have the best overall resistance to epidemic diseases, since they will have the best chance for the Atjuntja resistance to Aururian plagues combined with the Old World resistance to the Old World plagues. So that community could grow to be a large one.