Yep, you can join all the guilds, I recomend you do actually because I think you need to join them in order to turn in the test items.
You can only contract with one though, meaning you can only turn in items to one to get the special synth abilities/items.
You can switch guilds after you get the item/ability you want tho. I was contracted with the woodworking guild so I could get the lumberjack ability, then I switched to alchemy so I can get everything from there. The bummer thing is you loose any points you have with that guild when you switch, so make sure you spend your points before you leave.
You can only take one craft to 100, the rest to 60 (with the exception of fishing, you can take fishing to 100 also). You need to remember to turn in test items to the guilds when you get to crafting level 8-10 (18-20, 28-30, etc) or you won't be able to level over the 10 (20, 30, etc).