You can be a member of all the guilds at the same time and you can level all the classes if you want to.

The way guilds work is something like this:

At rank 20, you can join the guild. You will get 3 quests from the guild (at rank 20 when you join, at rank 30 and finally at rank 36). You can buy skills and traits from the guilds as long as you have enough Guild Marks. You get those marks from leves that are rank 20 or above. Once you have enough marks, you can purchase the traits/skills and equip them. As far as I know you can't use the skills you buy on other classes (for example you can't use Heavy Slash as anything but GLA itself) but you can use any trait on any class (the exception is that you can't use crafting/gathering classes' traits on fighting classes and vice-versa).

Acquiring guild marks is quite random, and it depends on the ranks of your classes. So if your GLA is rank 5 you can't get gladiator guild marks from any leve. If it's rank 20 or above you can get marks from leves that are rank 20 or rank 30. If it's rank 30 or above you can get marks from leves that are rank 20, rank 30 or rank 40, and so on.

I hope this answers your questions.