I'm not really sure what you can do with the 8lb dumbbells. There is a variety of Dumbbell assistance exercises you can do for preventative maintenance but they aren't really heavy enough to strength train with unless you tie them to your feet during Hanging Leg Lifts.
Most of these DB exercises are for the joints. Look up Diesel Crew on youtube. Carl Paoli has some DB exercises on GymnasticsWOD.com. Those exercises are assistance drills for Cross, Planche, Levers, and Invert Cross, Maltese and Victorian.
The exercise ball is fairly useless for the most part. You can use one to do back extensions or glute-ham pulls in a glute bridge. Ab crunches are ok, but let's face it they are bloody easy to do. They are handy for doing pike press HS off or reverse straight arm planks and bouncing straight arm planks or planche leans (get in plank, lean towards planche).
For MAG, you need a lot of upper body and core (abs and lower back strength). However, the nature of men's events such as PB and SR train the lower back very well.
So focus on doing pullups (over/under grip, wide, L pullups, wide L in over/under, BehindTheNeck, Front lever pullups), front and back lever, hanging leg lifts, L hangs, V-hangs, and skin the cats.:
Here is a basic programming guide:
The Fundamentals of Bodyweight Strength Training | Eat. Move. Improve.
Here is a series of articles concerning some basic gymnastics drills, rings and parallette strength, Handstand stuff:
Gymnastics Articles
Here is a lot of conditioning drills:
Conditioning Tips Drills and Exercises. Find sport specific conditioning for gymnastics. Will help in all sports There is also a WorkOutoftheDay which is ok, but not always gymnastics stuff.
If I were you, I would make a cheap set of paralettes (pushup bars) with PVC and a set of homemade rings to hang off your pullup bar. They can be as simple as some rope and pvc wide enough for your hands to molded rings made by warping pvc in the oven and wrapping them around a paint can to make a ring (fill the PVC with sand, cap with duct tape, heat for 10 minutes). Thread with either chain or rope.
GymnasticBodies.com is another website with a lot of information and a forum as is EatMoveImprove.
Beastskills is another good website with some tutorials for gymnastics strength.