Not at a normal serving. Rambutan grades D for total FODMAPs, D for excess fructose and C for polyols, so it is a moderate to high FODMAP fruit. Two or three fruits is where tolerance testing sensibly starts, and that is a more realistic portion than it sounds because each one has to be peeled and eaten off the stone. The distinction worth holding on to is that rambutan is a grade milder than lychee and longan on every FODMAP axis, not that it is clear.

Usually yes, and the grades back it up rather than just the intuition. Lychee is E for total FODMAPs, E for excess fructose, D for polyols and E for SIBO protocols. Rambutan is D, D, C and D on the same four, and it adds nothing new: it is A for nickel where lychee is B, A for oxalates, and clean on histamine, tyramine, gluten and sulfites. So it is strictly safer on the triggers lychee is flagged for. If lychee causes you problems at any amount, though, start rambutan at one or two fruits rather than assuming the milder grade means a free pass.

No. The seed inside a rambutan contains saponins and is not eaten raw. Some regional preparations roast the seed, but there is no reason to bother and no benefit worth the risk. Peel the hairy shell, eat the translucent white flesh off the stone, and discard the stone. Some varieties cling to the seed more than others, and a thin brown seed coat that comes away with the flesh is worth spitting out too.

On the available measurements, no. When researchers quantified hypoglycin A and MCPG across soapberry fruits, both compounds fell below the reporting limit in every rambutan aril tested, whereas lychee arils registered clearly measurable amounts of each. Rambutan is in the same botanical family as lychee and ackee, which is where the question comes from, but family membership is not the same as sharing the compound. Rambutan is worth moderating for its sugar and FODMAP content, not for this.