I have a few personal beliefs on how many skills someone can have and how skilled they can be on them.
First and foremost, I have an easier time thinking someone could be skilled in multiple combat abilities if there is a straight correlation between them. For example: Arcanima, Scholarly magic, and Allagan Summoning all use the same mathemagical baseline (delineated from Summoning, if I recall), so I can see someone being skilled in all of these with little issue because they're using the same baseline. It's all "good at Math" in the simplest terms.
Dark Knight and Warrior fall under this sort of thing too - not only are both about swinging big two-handed weapons, but they also delve into tapping into an inner power that needs to be tempered and controlled. And I'm sure the bow skills of an Archer/Bard can correlate decently well to a Mechanist in leading shots and aiming for weak points at a distance.
It's when the skill sets start moving apart is when I feel degradation will start to take place. And how many degrees removed also aids to that, in my mind. Someone used to a sword and shield might have a few skills that transliterate to using two daggers, but you'd figure they'd need a bit of practice (and often have to mentally shift) from having something big and bulky to block that incoming strike. And keeping up on your Monk training while also delving into unlocking a forbidden/lost art like Black or White magic would also likely leave little time for anything else.
And then, as you add more, I just start to wonder how they have time to actually socialize if they want to maintain "mastery" in all these things. If you're a Paladin, Dragoon, Scholar, and Dark Knight, I would expect there to be some give in that skill set - some of the skills being better than the others due to active practice - that I can jokingly compare to the degradation of spell damage for Flare and Holy. In fact, I'll quickly make up some arbitrary pseudo-skill percentages for my own personal amusement.
Focusing solely on one thing: 100% skill.
Study of two closely related skill sets: 95%(95%)
Study of two vaguely related skill sets (singular focus): 90%(70%)
Study of two vaguely related skill sets (equal focus): 80%(80%)
Study of two unrelated skill sets (singular focus): 85%(65%)
Study of two unrelated skill sets (equal focus): 70%(70%)
Study of three skills (all related): 85%(80%)[75%]
Study of three skills (two related, one non): 80%(70%)[40%]
Study of three skills (all unrelated): 75%(50%)[25%]
All of these numbers are completely and arbitrarily made up, of course. The idea is that taking on another skill hinders the "skill level" of the rest. Having correlation reduces that, while not having correlation increases it. And, overall, the ones who specialize more will - in the end - be more skillful in comparison because of it. Even if someone studies both the related fields of Scholarly magic and Summoning, their skill will ultimately be less than someone who focused solely on one or the other.
And all of this is using a baseline of the same type of person of the same age having spent the same amount of time divided over however many skills they claim to have. Adding in differences in age, bloodline, history, outside assistance... that makes the whole thing a cluster.
I'm... sure I had a point in there somewhere, but I think I lost it in favor of throwing out random percentages.
First and foremost, I have an easier time thinking someone could be skilled in multiple combat abilities if there is a straight correlation between them. For example: Arcanima, Scholarly magic, and Allagan Summoning all use the same mathemagical baseline (delineated from Summoning, if I recall), so I can see someone being skilled in all of these with little issue because they're using the same baseline. It's all "good at Math" in the simplest terms.
Dark Knight and Warrior fall under this sort of thing too - not only are both about swinging big two-handed weapons, but they also delve into tapping into an inner power that needs to be tempered and controlled. And I'm sure the bow skills of an Archer/Bard can correlate decently well to a Mechanist in leading shots and aiming for weak points at a distance.
It's when the skill sets start moving apart is when I feel degradation will start to take place. And how many degrees removed also aids to that, in my mind. Someone used to a sword and shield might have a few skills that transliterate to using two daggers, but you'd figure they'd need a bit of practice (and often have to mentally shift) from having something big and bulky to block that incoming strike. And keeping up on your Monk training while also delving into unlocking a forbidden/lost art like Black or White magic would also likely leave little time for anything else.
And then, as you add more, I just start to wonder how they have time to actually socialize if they want to maintain "mastery" in all these things. If you're a Paladin, Dragoon, Scholar, and Dark Knight, I would expect there to be some give in that skill set - some of the skills being better than the others due to active practice - that I can jokingly compare to the degradation of spell damage for Flare and Holy. In fact, I'll quickly make up some arbitrary pseudo-skill percentages for my own personal amusement.
Focusing solely on one thing: 100% skill.
Study of two closely related skill sets: 95%(95%)
Study of two vaguely related skill sets (singular focus): 90%(70%)
Study of two vaguely related skill sets (equal focus): 80%(80%)
Study of two unrelated skill sets (singular focus): 85%(65%)
Study of two unrelated skill sets (equal focus): 70%(70%)
Study of three skills (all related): 85%(80%)[75%]
Study of three skills (two related, one non): 80%(70%)[40%]
Study of three skills (all unrelated): 75%(50%)[25%]
All of these numbers are completely and arbitrarily made up, of course. The idea is that taking on another skill hinders the "skill level" of the rest. Having correlation reduces that, while not having correlation increases it. And, overall, the ones who specialize more will - in the end - be more skillful in comparison because of it. Even if someone studies both the related fields of Scholarly magic and Summoning, their skill will ultimately be less than someone who focused solely on one or the other.
And all of this is using a baseline of the same type of person of the same age having spent the same amount of time divided over however many skills they claim to have. Adding in differences in age, bloodline, history, outside assistance... that makes the whole thing a cluster.
I'm... sure I had a point in there somewhere, but I think I lost it in favor of throwing out random percentages.