Should I fertilize a succulent/cactus???

Yes, I suggest to use fertilizer during growing season, especially if the plants have not been repotted for some time as they already used all the nutrients from their soil. 

It can be the same fertilizer as for the other plants, but you must remember that cacti/succies need low nitrogen (unlike others plants), but they like high level of phosphorus. So any of cacti/succie designed fertilizer will be better. Use it diluted in half.

As you already know, cacti/succies don’t need lots of water, the same situation is with fertilizer. Overfeeding them will be just as dangerous as overwatering. So 1 or 2 feedings during the growing season is enough to make your plant happy.

I hope you will find my post useful 

Happy growing!