This is actually a really cool subject to discuss. As 8thed is my main for gaming, I will try giving some of my thoughts on the matter.
As far as Warhammer goes, having codexes/army books from different editions facing each other is a pretty standard thing. I would rather say that this is the norm and situations where all the books become updated during a single edition are pretty rare. Even when all the books become updated (like in 6th edition Fantasy of 8th edition 40k), there can be also additional materials which buff certain armies and change the balance between the books significantly. When considering 'power levels' of different army books, never had there been a situation where all armies were considered equally matched. To me this is all is very subjective, as the game is so complex and nuanced that pursuing perfect balance actually takes away from it. A lot has been said on the subject of balance through the years but the aspect I always like to bring up is that player skill is always a huge factor. Some people can do amazing things with army books considered ‘trash’, while other struggle to win with lawnmower lists. It’s better to throw down some dice and see how it goes, than to worry in advance too much.
When it comes to the 8th and 7th ed High Elf books I would say they were very close in raw stats and point values. The 8th ed has obviously more in it (meaning, more units), but in 7th the elite infantry was a bit more efficient. There were also more magic items in it. This was toned down in 8th, but at the same time a lot of very good options like the Loremaster, sisters of avelorn and phoenixes were added. I would say you're okay using the 8th one for the additional options.
When it comes to Skaven, the 7th ed book was an absolute powerhouse when it came out. The amazing magic, items and solid units made it a top-tier option for tournament goers as far as I can remember. Since Skaven relly on big units of infantry and warmachines, they transitioned into 8th ed very neatly as that's what got boosted in it. I have been regularly playing against them with various armies in the last 11 years (High Elves included) and the army holds up magnificently well against anything that 8th ed can throw at them. I would even say that apart from a few overcosted special characters there are really no bad options in that book (there are a few stellar though, which steal the spotlight).
So summing up - I would say the 8th ed High Elves and 7th ed Skaven are very well matched and in this case you have nothing to worry about.
