I don't play JP but I've looked into the mechanics enough and simulated enough battles to be able to give decent enough advice. They need to be at least 100 with maxed active skills. The Grand Score and extra CE slots are only useable in the Grand battles, but the +1000 atk. bonus is permanent. There are clear winners for the best servants to choose from a gameplay perspective, but there's enough of the items that let you change your Grand servants available to never regret your choice unless you're incredibly careless and somehow use all of them.

Grand candidates are all the servants that are 100+ with all skills at level 10

For the duels themselves, you HAVE to bring your grand, can be in the backrow, then you have one support from your friends and 4 servants that are the same class

With the grand buffs you can bring an AoE servant to a ST fight and vice versa.

Also, every time we get a new grand duel, we get 5 items to switch a grand so there's no real buyers regret possible

Are all servants eligible? Or only the lvl 100/120?

You can use servants of any level, but only a servant with level 100/120 and fully upgraded skills can become a Grand.

Are Grands the only one allowed in the Grand Duel (each class Grand event where you fight Musashi/Heracles/Olga, etc...)

I'm not sure if I understood the meaning of the question correctly, but you can use any Servants, not just Grands. However, you can only start a Grand Duel if you already have a Grand Servant; otherwise, you must obtain one first. Also, you can only use Servants of the Grand Duel class, meaning that in a quest like the one with Musashi, for example, you can only use Sabers.

I'm also assuming Grands Supports do not count towards "having a Grand servant of one class"? (Like taking "Grand Support Koyan" to access Grand Assassin Duel if I don't have a Grand Assassin myself")

Learning that you can switch Grands however, is awesome news.

Thank you for the clarifications

No, you need your own. And you get 5 compasses (the item for switching) every time a new class is released, so you should never run out.

What about the extra Grands? If you Grand a beast, can you only bring the two beasts or all of Extra I (or was it II?) ?

Also, can you Grand any two extras as long as they are different classes or can you choose from one Extra I and one Extra II servant?

It's any 2 Extra class servants, but only one per class, so you can't have 2 Rulers for example. Extra I has designated battles, and Extra II has designated battles. It's better to have one of each initially so you can max the scores. Your party is only limited by Extra I/II. So if you have a Grand Ruler, you can fill your party with Avengers, Moon Cancers and Shielder too.

Only Lv100 and skills 10/10/10 servants are eligible. Don't have to be 120.

For the Grand Battle(not necessarily a duel, as for Grand Lancer we had Indra and 2 Vajras as our examiner) the Grand servant MUST attend. So you need a Grand + 4 others from the same class + 1 friend. However you don't have to place your Grand in the starting members.

I think through before actually go grand them, and I don't really regret. I am planning of re-grand my Grand Berserker in the future, but it's something we've decided before.

The difference, especially in the Grand Battles, is big. Since Grand Berserker, most non-grand servants are basically useless as attackers. However, for Grand Saber, non-grand Medusa was quite strong.

For beasts, you need to beat all 4 U-Olga Maries and complete the medal thing.