I think all characters can learn either T or S skills, and thus, be either T or S orientated characters. With Lloyd, it's a bit tougher than usual to keep up, because some of his T and S skill names are very, very similar sounding.
To be honest, I'm not too sure HOW the ex-gems affect the T/S levels. I only remember reading that they DO affect. It takes experimenting, and that will cost you Ex-Gems, of course. It's been too long since I played. And I never paid too much attention to the Ex-Gems, honestly.
You know how skills are color coded, right? I don't remember the exact colors, since it's beena few months since I played ToS, and it was many RPGs ago. I think if a skill showed up as red (or possibly blue) then that meant that you were in the process of "learning" that skill. Essentially, you'd have to practice it's predecessor in the skill tree enough to learn the next skill. So, for instance, if you saw a red (or blue) skill name in Lloyd's list with the phrase "Tiger Blade" or "Beast" in it, you'd have to use the LAST "Tiger Blade" or "Beast" attack that you learned at least 50x in combat in order to learn the next one.
The wrinkle? Sometimes, depending on the rest of your skills and your current Ex-Gem makeup, it's possible to switch from T to S (for example) while still learning a T skill. That T skill could, potentially, never be mastered for the rest of the game. That happened to me with a few characters (namely Zelos, with a couple of healing spells he just never mastered.
What I'm saying is that using a T skill 50+ times in combat in order to master the next T skill with NOT work if your character shifted to an S-type character before he could master said T skill.