Not sure if you are familiar with the terrain in the Highlands. Most of the ground is wet, if not completely sodden. In the winter, this means the ground can be frozen into very solid ice. When not frozen, it makes for a cold, very wet surface to camp on, so I normally look for a local high point, top of a slight rise or even a hilltop, as these are often the only place that is relatively dry and flat, and in summer a breeze is desirable to minimise midges and condensation.
These higher points seem to be a thin layer of soil and plants over bedrock, and the soil has lots of rocks mixed in it. The chance that you can place a peg exactly where you want it is quite small. Once you do get a peg all the way in, it usually holds OK, and you will see if its dodgy. Often there are few or no movable large rocks to use as weights or anchors, though this does vary from location to location.
For pegs, I would suggest a mixture, I tend to use old Easton tubular type pegs, or groundhog style pegs, circa 200mm / 8″ long. A thin nail/wire type peg might work in some places, I haven’t tried. If you have a mix of types, you can use the most suitable for the most important points. Extra long pegs might help a bit if the ground is soft, but if it is stony, its just more likely you hit an (immovable) rock before the peg is all the way in.
What I think is more important than the type of peg is to be flexible in the peg location. I would try to have long-ish adjustable cords on the key peg points, so you have a good area to find a good peg placement. The cords are also easy to wrap around a decent sized rock – if any are available.
I’d also suggest decently long guy cords as the main way of holding the tent down, as you have lots of flexibility in where to place the peg. The wind tends to change direction quite a bit, so take enough pegs to secure all sides of the tent.
If it’s “coming on to blow”, the best strategy might be to prioritise shelter from wind and suitability for pegs over comfort. In other words, camp on a wet deer-trodden bog or similar. Here, wider/longer pegs might hold a little better, but in this kind of wet, soft ground almost any pegs should be OK.