I investigated it even further a moment ago.

So I've placed one of units at the beginning of the crusader, replacing Highland Clansmen and added a new one as 257th unit in the file.

I expected everything, but suprisingly it was all right - the new units used its own icons and animations, so did unit nr. 258 at least I thought so.

I've tried the new unit in the campaign and ... it appeared as possible to produce with its own icon and description so I have recruited one and it appeared as... Highland Clansmen...

I believe that any combat unit after 256 will by default be recruited as the first unit in the original crusader unit file.
I am not really interested to try if I change the first unit in the original file to a different one than the Clansmen I will get it as the new default unit - clearly it has no use anyway [EDIT: writing this post I think I have found one, perhaps quite intriguing...] , except in custom ( maybe MP too ? ) mode so it is good news to such mods, the bad ones are there will be nothing more in the campaign than 256 combat units.

Anyway it is excellent that we have those non-combat slots to explore now.