Senate rules since the 1970s (when the filibuster was set at 60) require 66 votes of all senators to effect a rules change.

This utilizes the majorities ability in any parliamentary body to set the precedent or rule on a point of order. hence the 'nuclear' option.

I assume that the ability of the Majority leader to declare that only 50 votes are required is limited to Senate rule changes?

If it is limited to Senate rules changes then it's irrelevant. The Senate rules are, after all, the rules. For example:

Majority wishes to pass a bill that the minority doesn't agree with. Majority wants to change the rules on that vote to get it passed despite minority.

Minority says they can't because the Senate rules forbid it.

Majority changes rules from forbidding it to allowing it.

Granted, that's a simplistic way of looking at it. It would seem to me that rule changes should have higher requirements than simple majority because then the majority can change the rules whenever they wish.

The power of precedent and fear of being in the minority is what generally fights against this. Rule changes that drastically favor the majority will hurt them when they end up in the minority.

I totally agree.

One of the very few enumerated power in the Constitution is that the Senate and House get to make their rules however they see fit. What usually happens is that at the beginning of the new Congress the first order of business for each body is to set the current rules. Part of those rules are how to change the rules during the current.

It's common for these to be passed unanimously.