From my point of view....
A few things I have considered:
Each profession has a limit to a number of skills in specific categories that receive the ability to be scaled.
Only 1 scaling option per 3 ranks in a skill (maybe higher like every 6 ranks?), along with a max limit of 1 per character level or every so many levels.
Scaled options for below rank 10 would be nothing really special, whereas the options for above 10 ranks are more appealing and beneficial.
There will be scaling options that will not add or subtract any modifiers and are more for the role playing aspects but are requisites for higher level scaling options.
Using development points to acquire some scaling options.
Some pre-reqs might be specific talents, multiple skills and ranks in those skills, character level requirements, etc....
Limited # of skills that can be scaled? No. If you have invested the ranks, you can scale it. Same as with spells.
Limited scaling options? No. If you are at a certain skill level (# of ranks) then you have available any scaling options that can be covered by that many ranks, including the ability to combined scaling options (similar to spells)
Degree of "special" for higher ranks - definitely, but there is nothing special about 10 ranks or any other level. As the number of ranks go up, the scaling gets more powerful - and combining multiple scaling options (multi-attacks, multi-parries, multi-facings, etc) in one attack can give you an incredible action that round, but the scaling costs stack and they bring penalties with them.
DP can be used to add ranks.... once you have the appropriate # of ranks, you have the ability to do the scaling option. There is no additional cost required.
Pre-requisites? None.
Now, keeping in mind all of the above, the scaling costs are not going to be cheap, but when fully implemented it allows for that Conan-like character to wade into battle and wipe out multiple foes every round so that there is still value to adding ranks in a skill once the diminishing returns have dropped to minimal levels. This makes it more beneficial for characters to develop non-spellcasting skills instead of blending into that same multi-profession standard that many HARP characters do once they reach high levels and have all of the talents and effectively max'd out skill progression and have boosted their stats up to 95+ across the board.