You can build skyscrapers in area's with skyscrapers around it, so it won't look like a random high build in the middle of a neighbourhood with only houses. If you aren't sure if you build a skyscraper on your plot, you can always ask staff.
The biggest factor when determining what can be an apt building is if it meets city standards. One part of this is if it fits the area it is in. Like you cant build a big tall building in the middle of a bunch of small residential ones. Another part is the appearance of the building itself both on the inside and out. You can build a tall building in the middle of the city where other tall builds are, but if it doesn't fit in, it won't look good. My suggestion is if you find a plot you think could be an apt building, call a srmod or admin over and ask them what they think. Most importantly, hire an architect to build for you, as their work is often pretty amazing and will provide a higher likelihood of the plot meeting city standards. Hope this helps!