It’s relatively easy to learn the basics of Kung Fu with some practice. But it is very hard to master Kung Fu at the professional level and beyond. Shaolin monks, for example, dedicate a lifetime to achieve mastery of Kung Fu and they work at it for nearly all of their waking hours.
Yes, you can have multiple black belts in different martial arts. In theory there are no limits to how many black belts you can earn. As long as you become good enough to qualify for a belt in each fighting discipline, you can earn another belt with any others you might have.
All black belt practitioners can fight but not to the same extent. If the black belt is earned as a symbol of excellence, it’s fair to say that he or she can really fight. If the black belt is earned for mere competence, then he or she have achieved a certain level of mastery but may or may not ‘really’ fight.
There are Kung Fu masters in MMA but due to the nature of MMA fights, it may mean that you rarely see them practicing Kung Fu in the MMA ring.
You can spar with training gloves. Both sparring gloves and training gloves protect your opponent but training gloves also give more protection to your hands. This means that sparring in training gloves provides less real-world training than sparring in sparring gloves, which are meant to mimic real fights as closely as possible.