Yeah I agree, while Spanish purely-military successes are quite plausible and would not be surprising at all, their presence comes under question the moment they lose mastery of the sea and fail to deliver supplies or reinforcements for a few months (something that Portugal, Netherlands, England, France, possibly Denmark or Sweden or Ottomans too would be only to happy to assist with). Look at what happened to the French troops in Arakan.
Also, any comparisons with late campaigns in Yucatan are really off-base. The individual Spanish forces involved in Yucatan in the late 17th c. were in the double digits of soldiers, most of whom were locals and not Spanish, most expeditions were coupled with trade caravans or missionaries and were basically more civilian than military, and on top of that Spanish governors were actively or passively undermining each other's attempts. It proved to be enough.
The level of resistance from the SEA kingdoms would be incomparable. It would be armies of tens of thousands, like someone already said. No force of fifteen guys and a dog would be able to make the slightest dent on that. The Spanish would need to maintain several hundred to several thousand soldiers in top fighting condition to back up a friendly local kingdom just to have a chance of holding on.