If you have a skill at 10, then a potion will do nothing as they cap out at 10. Abilities on the other hand can go another +2 above the max level, like 7/5 or 8/6 and gain extra stats. Some can only go +1 above the level but you can check to see under the abilities section.
There is a cap. I'm pretty sure +2 over the max number is the highest they can get. That being said, I'm pretty sure that the gear stacks, ie: if you have 1 point in a skill with a maximum of 6, and you have gear that grants +4 to all your stats, you will get the benefit as if that skill is raised to 5, but if that skill was already raised to 5, your skill will be at level 8, not 9.
I can read your minds what little there is. I know what all humans think. Nothing's changed in 1800 years. You can't be saved. -Orochi, KOF97
Many abilities can go +2 over their natural total. Abilities will show a 'next level' amount once over-leveled to let you know if they can go even further. Note that this only shows on the abilities menu, and not the level up screen(which only handles the points you can invest). There are exceptions; most notably all the 'Weaponry' lines cannot go past 3/3(extra Fate generation). This means you can leave them all at 1/3 and use gear to max them out instead.
"I'd rather betray the world than let the world betray me." -Cao Cao