Yes you must have the required craft to meld materia to the item. You also must be of appropriate level and have the required catalyst. If you meet those 3 requirements, you can meld materia to that item. The first materia is 100% successful everytime. Each additional materia after that has low chance of being successful. I heard putting a 2nd materia is around 20% success rate. Someone else would have to confirm or deny this. I have no experience with putting more than 1 materia on at the same time. If you hover over an item, it'll tell you all the info you need to know. If its meldable, it'll say what level is required to repair and what level is required to meld materia to that item. If you don't meet the requirement, it will appear in red and if you meet that requirement, it will appear in green letters. I hope this helps!
Consecutive melds can have a widely warying success rate... anywhere from 2-50% from personal experience. It has to do with the quality of the item you are melding materia on to (whether it is NQ or HQ) and what materia is already on it (not only does the Tier affect it, but also the bonus it gives, i.e. an item giving STR+1 vs an item giving STR+2, even if they are both Tier I). As a rule of thumb, if you plan on melding multiple Materia on to an object you always want to put the weakest Materia on it first to maximize your chances for additional successes.
Say I want to triple socket a Cobalt Cuirass with HP Materia (not that I would...lol) If you have a Bloodthirst Materia Tier III HP +50, a Tier IV HP +65, and a Tier IV HP +70...you would want to use the Tier III first (because it is the lowest Tier) then attempt to attach the Tier IV HP +65 (because the bonus is less then the other Tier IV), then the +70.
Also, when attaching two different Materia to an object you get a bonus (melding multiple Materia of the same type will always result in a lower % chance versus another type of Materia of the same Tier).
Hope this info helps you out. Edited, Dec 27th 2011 10:16am by StateAlchemist Edited, Dec 27th 2011 10:17am by StateAlchemist