Well first of all, Zelda is not an RPG, rather some sort of puzzle solving action adventure.
But I will not go into what makes an RPG in this post.
Secondly, your question is really a design question. So there's no definitive "that's the way to go" type of answer. But I can give you a few different examples, highlighting their pros and cons (TLDR at the bottom).
Example no. 1 will be the classical SNES (J)RPGs like Lufia or the early Final Fantasy's. In those games, you only control your main character in the levels. But you do control all of your party members during battle. Mages will not be able to wield swords and will always come with their default weapon. (In Lufia, there's even a mechanic/scientist who can only wield wrenches). Likewise, a fighter will only be able to cast so much magic (some form of your suggested "jack of all trades", I suppose).
As a result, there is no AI involved, whatsoever.
Consequences are, you, as a designer, will not have to care about separated parties, meaning, there is less balancing work to be done, no story branches for every possible combination of parties and so on. But, as you do want to give your characters, well... character..., you will have to do a lot of strict writing, lots of dialogue and so on. This can take some interactivity out of your game but, at the same time, make for stronger stories. The weapon restrictions will create more work for designing classes (and items, obviously). But, as players control their whole party during battle, this puts a lot of strategy into the game, requiring the player to think and plan ahead. And each battle will become a brain exercise for the player. The decision is yours whether that serves your game or not.
Example no. 2 will be "Dragon Age: Origins" which is a kind of hybrid between action and turn-based (although most players play it action-wise). Now in this game, you can control everyone of your characters. But there's also (exceptionally good) AI involved.
Consequentially, that led to me, focusing on my main character because my other party members did such a great job. But because each character will only have really few skills (at least in the beginning), that removed all of the strategy and thus became very boring very quickly. Meanwhile, of course, it sped up the gameplay (which may or may not be desired). Yes, the game really had a lot of issues but that won't lead to anything here.
Example no 3. is, again the old SNES stuff, the "secret of ..." series (most commonly known, probably being "Secret of Mana"). You can control any character of your party. But the game is action-based enough so that having AI in the game is essential. Also, just because of the action, fighting will still not get boring, even after hours in. But that's just to give a counter-example to no. 2.
Example no. 4 will be "Divinity: Original Sin". This one, again, is purely turn-based. Here, you don't have your typical RPG classes, but instead level each character the way you want to. That way, you can create all sorts of combinations like battle mages (mine, for example, wielding two axes), stealth archers and so on. In "Divinity", you control each of your characters yourself. Eventhough there's a cap of four characters at max, you can have multiple parties and re-arrange them anytime as well as characters not being in a party at all. The only sort of AI acting here is "follow your current party leader while not in battle".
Now, this will give the player a lot of freedom. For every battle, every quest, every task and every single problem, there are a thousand ways for the player to approach and solve them. Players will have to carefully plan their party, come up with their own unique strategies and execute on them. This is true, not only in battle but also in the field or in hubs (townships, etc.). Which character will you pick to talk to which NPC? If a character is too weak to break the door open, is there anothere character to try using lockpicks? Or can you teleport another character into the room to open the door from the inside? Maybe there's someone to talk to, that can help open the door. But maybe he/she will only help his/her dwarven friends. This is probably the most RPG-ish approach but also the hardest to implement. Because you will have to account for everything. Is the puzzle solvable by any type of character build? Can the NPC react to every type of character (gender, race, profession), appropriately?
To summarize and wrap it all up (TLDR), your questions' answers really depend on the type of game you're making and what goals you're trying to achieve. So nobody will be able to give you a straight answer right away without having in-depth knowledge of your game. But I hope, pointing out the examples above will give you enough to consider to help you find your answers yourself.
Cheers 
