PHYSICAL.

The stones vary considerably in shape and size, from a small boulder through man-sized, to 10 - 12 feet (3 - 4 metres), from as-found, to laboriously shaped and smoothed, to elaborate crosses which are fully three-dimensional.
The main stone used was sandstone, although there are some notable exceptions.
Some are basic scored designs (incised), while others are carved to give more three-dimensional figures (relief-carving).
Some are richly decorated, and some have as few as one or two symbols.
In many cases, it looks as though the masons were working to templates, as carvings crash into one another, looking as though the sculptor could not adjust the size of of whatever he was working on. Either that, or precision was not a virtue in Pictland.
So-called Class 3 Stones tend to show more careful work, but we know from the Book of Kells that "Celtic" art was carefully worked-out before drawing commenced and there is no good reason to suspect monkish masons would not have followed these practices, too. Even so, the precision of the work varies, and there are many examples of mismatches, especially in complex designs.
Occasionally, it looks as though a previous stone has been "Pictified" by the carving of symbols. Certainly the
Newton of Collessie Stone gives that impression.
Interestingly, there there are many standing stones in the Pictish area that have not been "claimed", and it makes me wonder which was more special, the Pictish stones or the ones that were left untouched. Perhaps it implies that the plain stones held a significance too great to interfere with, or that the Pictish remit did not run deep in some communities. It might just be that the sculptors found it easier to work on the flat, and the standing stones of previous times were too awkward to work on, since they are usually "as-found" boulders.
There are clusters of sculptured stones, and you have to wonder when they were collected together.
The Isle of Iona is reputed to have had 365 crosses, presumably representing a saint for every day of the year. Perhaps lesser Christian communities had smaller collections of mementoes?

Although many of these stones are now isolated, it is hard to believe that there was not some sort of settlement attached to them, especially one like St Orland's, which is richly carved, and must have represented a good deal of work. It stands forlornly on a bare knoll in an area that is barely cultivatable even now. Why was it put there? Two or three miles in any direction there is plenty of land, high and dry, and suitable for habitation.

What dictated the placing of these stones?
Was there a stone there already, that had to be displaced?
Perhaps those better versed in the theories of ley-lines and astronomic observations in the Professor Thom tradition, might be better able to shed light on that aspect.
Certainly, there are many instances where Christian missionaries took over pagan sites, either to destroy them, or re-use them.
Neither the Picts nor their stones existed in isolation from the rest of the World.
At present, they are like a scene in a jigsaw puzzle where we have lost the picture. We need to build outwards from the fragment we have, and inwards from the edges to see where it fits.

CLASSIFICATION.

Back in 1903, Anderson and Allen's The Early Christian Monuments of Scotland was published [ Hereafter referred to by the usual ECMS, for short.] It rapidly became the standard work on Pictish studies, due to its thoroughness and the scope of the material they studied. Although it became a bit of a strait-jacket, it is still a valuable starting-point for Pictish Studies, as the bulk of it is still valid. It also contains information that is not otherwise easily available.
It was Anderson who proposed the system:-
CLASS 1 - Incised symbols only, on boulders, slabs, pillars or rocksurfaces.(includes caves).
CLASS 2 - Relief symbols and "Celtic" ornament on erect cross-slabs, or recumbent coped stones.
CLASS 3 - Relief sculptures with "Celtic" ornamentation on erect cross-slabs, free-standing crosses, or coped recumbent stones, without Pictish symbols.
He also split them into three time-zones. Class 1 - 7th and 8th Centuries CE, Class 2 - 9th and 10th Centuries, and Class3 had ended by the 12th Century.

Apart from those with crosses on them, or which are actually cross-shaped, there is no reason to associate them at all with Christianity in any of its forms.
There is no reason, either, to ascribe them to these periods. The Victorians believed in "progress". Complex things proceed from simple things. Not so. Chippendale has been and gone, and there are still people who cannot assemble a tacky pre-drilled flat-pack coffee table.
It might be wiser to ascribe the differences to cultural preferences.
I suggest we abandon the "Christian Monument" bit for all but the indisputable crosses.
I suggest we ignore the time frames for all but the "historic" missionary works that can be tied to external sources. The incised stones could be any age at all, as could the non-Christian ones. It has to be conceded that the crosses are more than likely the results of missionary work from Iona, or Whithorn, although some might relate to work by the mysterious Culdees who pre-dated the Roman Catholic missionaries, and who were eventually absorbed or displaced by the Roman orders of monks such as the Cistercians.
Perhaps all the carved stones are roughly contemporaneous. Perhaps non-Christians liked the fashion for sculpted stones, and adapted it to their own requirements. In later centuries, free masons did Church work and secular work, and there is no reason to expect their forebears were any different. Maybe the difference in quality of carving is down to the skill of the sculptor, or the level of payment offered by the patron. After all, there are originals of value, and cheap inferior copies, in all aspects of life.
Whatever.
The point is, we have to go back, unfettered by Christian propaganda, to see what is proveable. The Victorians were vigorous and forceful, but it doesn't necessarily make them right.
We need new questions to frame new theories.
The old ones have patently failed.