I'm pretty sure the Ottomans did not participate in this battle. I think the misunderstanding may stem from the fact that Portuguese sources called both Mamlukes and Jannisaries "Rumes."
The Ottomans hadn't conquered Egypt yet by the time of the battle of Diu. It was the Egyptians that sent a fleet to stop the Portuguese. The Portuguese were already stomping muslim trade on the Malabar coast prior to the battle of Diu, in the Persian Gulf too for that matter. The point of the battle was that Egypt joined with Gujurat and Calicut(the two main rivals to the Portuguese on the western coast of India) to put a stop to all the havoc the Portuguese were wreaking on their trade revenues and of course their attempt failed.
It should have been manned by Mamlukes and incidentally many of them stayed in India and continued to fight the Portuguese after the battle of Diu with Gujurat and Bijapur. In fact many of these exact same Mamlukes, who fought the Portuguese at Diu, also fought them in the battles Albuquerque fought with Bijapur over Goa.
Here's an excellent article about it in English.->
http://www.dancingwithdolphins.com.au/discovery/ Gwalcmai gives the Portuguese version of it in his post.
That would be Meliqueaz. He held back some of his forces because he saw his side was doomed. He didn't want to piss the Portuguese off as he realized he would later have to deal with them. But he didn't actually switch sides.
The battle was pretty much a rout. I don't think the Egypt/Gujurat side had much of a chance. Maybe if the Portuguese fleet was more poorly commanded it would have been a draw or something like that. I doubt Egypt would have been able to drive the Portuguese out of India had the Portuguese lost. Keep in mind they didn't last much longer themselves as they were soon conquered by the Ottomans. No doubt it would have badly hampered the setting up of a Portuguese sea-borne empire there though. Keep in mind also that the Portuguese naval tech was quite a bit superior to what they were fighting against at Diu. The real issue was that the Gujuratis had some very large ships and just the sheer masses that they were fighting against.
One of the more interesting things about the Portugueses' wars in the East Indies is that they consistently faced off against Mamlukes and then later Janissaries or rather ex-Janissaries turned mercenary. Most of the more powerful Muslim states in the Indies employed them. They were their crack troops and often one of them was the general commanding the opposing side too.
Another way of looking at the conflict was that smaller numbers of Europeans on both sides faced off against each other accompanied by Asiatics and Mestizos in battle after battle for about 150 years in the Indies.
Another interesting thing about this conflict is that the Portuguese literally did face off against Jannisaries in numerous battles. In fact usually larger numbers of Jannisaries with a larger number of Asiatic auxillaries than they had too and they beat them many more times than they lost to them. So one could quite accurately say the Portuguese soldiers were better soldiers than even the Jannisaries were, even at their peak.