Just regarding that last bit, I hope there's not that many giants, their country would have to be MASSIVE and so far we have yet to see a giant use haki. You'd think at the very least Hajrudin could use haki, being in the new world and a wrecking giant pirate crew captain.. Then you have all the giants in the marines that are vice admiral which were suppose to have some level of haki, but yet none have shown to use it, which fair enough that haki was introduced after the time skip but Bastille hasn't shown using either observations nor armament.
They'd be quite OP if they were able to use either haki or DF's, so I'm sort of hoping that they just have pure brute strength and robustness of being, well, giants aha
It has already been confirmed that there are several islands containing giants and that Elbaf is merely one of them. However they are the most known because they scream the loudest if you want to put it that way. This comes from the mouth of Jaguar D Saul, a non-Elbaf giant. So overall seen giants might be rare, but not that particularly low in population from a relative perspective. Frankly saying a few hundred individuals for Elbaf is rationally speaking way too low, it makes far more sense to speak about several thousands of individuals as in a global perspective, no matter how big an individual is, a species that consists only of a thousand individuals is on the brink of extinction.
We've also learned that according to Hajrudin the Giant Warrior Pirates of Dorry & Brogy are quite famous amongst the Elbaf giants. The average pirate crew at least has around a dozen individuals and Hajrudin himself said that currently his group of giants 'only' consists out of 4 people. We also recently learned that Elbaf has a prince, meaning there's a king, it's a kingdom and it has a professional army. So we have some indications of their notion on what many is and that they have quite a bit of divisions in their society. This is not possible if you only have a dozen individuals. My estimation of a few hundreds is like the lowest of the low. Anything under that is not even sensible.
That such a relatively large population requires a large island is really not an issue as Oda has bend such practical issues to his own liking before. In particular FI was kinda defying logic.
That giants can use Haki is almost a 100% certain as there is no reason as why they shouldn't as Haki is present in all living things. That we haven't seen a giant using Haki makes no difference and normally every vice-admiral and above should be able to use it, so Bastille should know it. In fact it makes quite a huge difference if giants can't use Haki as then they are pretty much huge ass targets, which was clearly shown with Oars during the War of the Best. Not all will be able to use it, but their primary fighters definitely will.
Up to this moment nearly any kind of large group the Strawhats interacted with had one or more DF users. It would be quite surprising if none of the giants would be one. The fact that Hajrudin participated in the Corrida tournament to win the Mera Mera no Mi with the intention of eating it, indicates they very well can eat DF.