I have a couple of Ulua's and have used a number of reels on them, ranging from Saltist 40/Torium 20/332 size to Newell 533/Trinidad 40 size. Here's some things to consider:
1. Will it fit? Depending on how it the rod is wrapped and the reel's clamp bolt spacing, the reel may not fit on the rod. I barely got a Saltist 40 onto a factory-custom Lightning Ulua. It scraped up the X-tube shrink wrap a bit. The Torium fit a bit better since the loosely attached clamp bolts allow for a little more adjustment. OTOH, the 533 fits easily since the bolt spacing is farther apart. You will probably need longer than stock clamp screws for it, though. Same deal with the Avet JX/LX and larger - wide clamp bolt spacing, but the studs may be too short to clamp.
Another option here is to make sure the handle section is done with thinner cork tape or tuna cord, or 3M Grip Tape. I have another Ulua with just the 3M tape and a Torium 20 fits easily. Just have to be careful about over-tightening the clamp screws and crushing the blank since there's less cushion.
2. Is it too big to handle? The de-facto standard reel is the 533. But if your hands are small or if you have a short thumb, casting a 533/Ulua combo might be next to impossible.
A word about Newells. I use a P533 on one Ulua and put whatever fits on the other. I like the 533 for 50#, but not much less. Would rather use something smaller for 40# or below, as the 533 has eaten 40# before. And if you're fishing Cedros, there isn't much reason to fish less than 50# on the long rod. If the boat you go on is more on the "old school" side like the Shogun, they may have a decent stock of Newell parts in case a Newell has to be rebuilt there on board. They all know that Newell drags fade, screws come loose after a hard day of casting and grinding, and that plastic parts occasionally fail under extreme conditions. If you're the hot-rodder type who isn't concerned about tearing down and rebuilding a reel, then go with a Newell for the performance edge and lighter weight. If you'd rather go rock-solid and not think about having to tinker with a reel on the boat, go with just about anything else.