Generally sky worlds don't have ruins but in this case we had to find a way to provide cave spiders and bats for the kill quests. I didn't want regular ruins spoiling the atmosphere of the skyblock so decided to put bats/cavespiders inside of a floating island to make it seem more natural. Having them predominantly enclosed in bedrock provided a security to stop players digging down for safety and finding themselves falling into what was originally the void below. We've since changed it to water so that we could retain the gravestones with some assurance players would be able to retrieve their goods again. Clearly, I'm not inclined to spend time now changing the bedrock as it would still be frustrating falling into the water.
As to where they are, we've thoughtfully placed two islands fairly close to the platform and you can see them easily. One is a regular island and the other contains a hidden ruin. I don't think it would be fair to make players travel too far just to find some slimes, bats or cavespiders. These aren't the only placed ruins by any means. I believe in rewarding adventurous players with interesting stuff to find if they can be bothered going out and looking for them.
There are absolutely NO ruins in the Nether. They can cause too much lag there. You actually don't have to spend that much time in the Nether. You need to gather some of the wood native to the Nether along with a few other items and then decide how you want to proceed, be it spending most of your time in the skyworld and only returning to the Nether when you require something in particular, or perhaps building a more substantial base in the Nether and growing most of your supplies there and transferring them into the skyworld as required. There's actually a lot of flexibility in the way you can approach your build.
One thing I'd strongly recommend for every player is to LOOK before blindly lashing out at a mob from the nether. NO pigmen will attack you unless you attack them first. If you hit one, all the rest within the vicinity will gang up on you and take you down, and I don't mean downtown to buy you a soda.
One thing about Elder is he likes a greater challenge and has told me he usually has mob griefing turned on which allows blocks to be blown up. We have turned off that feature for the skyworld to preserve players sky platforms. The first skyworld video that Elder made also had 5% of pigmen starting off enraged and that's why he had so much trouble surviving in it. I've yet to see his latest outing so don't know whether he's played it as it came or changed anything.
One last thing, since you're clearly a curious character, I thought you might like a sneak preview of what the new saves Template looks like.
Cheers