Is the 88mm good for bass🤔🤔?

That’s why I use them. I play bass and guitar and they work beautifully.

I got the green but they are very loud strumming, can you recommend something else? I play unplugged

You need something thinner if the green dunlop is too harsh for you. I assume you are playing acoustic? If so I would suggest the .6mm orange dunlops or .5mm red dunlops. If electric id try the .6mm orange or .73mm yellow. The yellow isn't THAT much thinner than the green though....

I just bought a Yamaha Classical C40 II on Amazon, did not come with Dunlop picks like It was supposed too. Anybody know what Gauge is best to use?

As a beginner, you may want to try light. Around .5mm or so. But there is not right or wrong pick to use. It is totally and 100% a personal preference. Just try a bunch of different types until you find one you like best.