Link slider with 1 keyframe to adjust the Y value while maintaining X value

Scale values are in an array like this: [50, 100]

That translates to x at 50% and y at 100%.

You can pull the existing value from a keyframe using the value method. This is how you would tell an expression to look at the existing value for x and give the Y value 75%.  [value[0], 75];

A slider only has one set of values. You can extract the value of a slider easily by just using the pickwhip to attach a variable to the slider by simply typing y = and then dragging the pickwhip to the expression slider and closing the line of code with a ;  If the slider is attached to the same layer you get something like this:
y = effect("Slider Control")("Slider");

You can also so simple math using  +, -, *, and /

If you want the slider to modify the existing y keyframes then you would write your expression like this:

y = effect("Slider Control")("Slider");

[value[0], value[1] + y]

If you wanted to control the Y value directly with the slider and ignore any existing values for y the expression would be simply:

y = effect("Slider Control")("Slider");

[value[0], y]

That should get you started. The Search Help field in the top right corner of AE will point you in the right direction if you simply type expression slider and press enter. It's pretty easy to figure out basic expression language.