heavy cavalry engaging heavy cavalry

[Domen]

A slow attack could be delivered in a tight and regular formation, a charge at speed, a la outrance, unavoidably led to an opening of ranks as individual horses differed in their top speed.

Not necessarily - a well-trained (in terms of unit training not individual skills) and well-knit unit could charge at speed equal to maximum speed of its slowest horse. In such case, a unit could keep its ranks close, no horse would be left behind. Modern horses can also be trained to "keep the line" even when advancing at high speed, as can be seen for example in the movie scene linked below (move to 5m 50s of the video):

http://www.youtube.com/watch?v=qnklW0NXxFA#t=05m53s

Of course here you can see this in small scale, as only 3 horses are riding "knee-to-knee" in this scene. But if one line of 3 horses can do this, then lines of 6, 9, 30, 90, 180, etc. horses can do this as well, since as we know "management of many is the same as management of few. It is a matter of organization."

===========================================================

BTW:

It was a good idea, but problematic to implement, to pick horses with similar physical abilities to each unit.

===========================================================

In contrast, 18th/19th century European cavalry were usually only two ranks deep because of the threat of fireearms, and I am not sure if they rode knee-to-knee (see eg. Keegan p. 159, or better one of the experts in Napoleonic warfare who he cited).

Indeed, but this was not always the case, there were numerous exceptions, for example:

http://www.battlefieldanomalies.com/eyla...charge.htm

It is my opinion that the French cavalry adopted a en muraille formation, in great depth. The en muraille deployment consisted of each regiment forming its squadrons in line immediately beside each other. There were no intervals between each squadron, and they rode forward boot to boot [Nosworthy]. The front of Murat's column would therefore be approximately 300 meters wide and, allowing for all of the reserve cavalry and the Guard forming one behind the other, and only minimal space between each regiment, would be some 900-1,200 meters from the head of the column to its rear. Indeed just such a formation is depicted by one of the more down to earth battle-painters, Simeon Fort in one of his Aquarelle's, "La bataille d'Eylau", (Musee de Versailles). Here we can see that, given the conditions of the field, and just as importantly the condition of the men and horses, that Murat used his troopers like a battering ram, or bulldozer, to literally smash his way through the Russian lines. Not only this, but when we take into account the fact that trumpet calls would be difficult to communicate to each squadron over an extended distance owing to the wind, and that officers orders would not be heard due to the same factor, then this type of tight grouping would make control and command far more reliable. At a walking pace the French cavalry would not over-exert their already much fatigued mounts, and the fact they were attacking Russian infantry and cavalry who were "masking" their own artillery as they advanced after the defeat of Augereau's Corps meant that they only had to receive the fire of Russian muskets, and possibly not much of this either when we reflect on what the weather could do, not only to these weapons, but also to the cartridges and powder used to make them effective.

In total 52 French cavalry squadrons participated in that massive charge.

And here is Simeon Fort's painting mentioned above: