A lot of newbie questions

Just play this for a couple of days, quite a few questions to ask: thanks for those who answer

1. What is the use of gold and pink jellies? It seems all jellies have some specific use but I cannot figure out

2. Where to get evolve and dragon jellies?

3. What is the use of C+ red sweet? I used it for exp but did not increase too much, unlike red sweet

4. When boosting skill levels, does it have to be the same? For example, can I use fireball on flame ball or fireball +?

5. Looks like a lot of R cards (or even R+) are totally useless. What do we usually do with them?

Thanks for pros : 0