Any advice would be appreciated

I want to have better communication with my system but I don't know where to start.

I also want to know how to use my alter's strengths. (If someone's social I'd like her or him to be out while in social situations.) Stuff like that.

I would also like help with depersonalization/derealization (I think it's called) because most of the time I don't feel like this body is mine, or that life is real.

Any help?

I tried getting parts to come out when it best fit their talents, but they simply come out when they want and can.

Communication begins with accepting all parts, loving them, no judging and let them be themselves.

Ditto what Tylas said.
Try asking questions, get to know them, they may or maynot answer, with trust they may be willing to discuss/share more. Let them know about the here and now if they've been stuck inside and unaware.

When a part is triggered, I'll ask inside for another part to come out instead.
So say an anger one is out front, I'll step aside and let that part speak, but if I see him getting too worked up, I'll ask inside for a calmer part to come forward so my angry part can express himself without blowing up.

Getting to know yourself will increase cooperation, between host and parts, and also let you know about how each part works, I have checks and balances, if one's going too far I call out the balancer part. Lots of juggling, and very exhausting but gets easier with practice.

With DP, I stay away from alcohol, (unless I want to feel it for fun, but usually it's annoying to find your awareness slipping from/not-in-line-with one's body, and it takes extra work to stay grounded). So grounding exercises, foot shaking/knee juggling really helps and is a little subtle(looks adhd-ish, but most people don't notice), and keeping stress in check helps with DP(and DR).

For DR I mostly accept my reality will always be fake, and try not to dwell on it as it makes my DR worse. I also stay away from movies that trigger it, unless I'm in a good mood where I will watch it, and get swept up with the movie, but then leave it(DR) when the movie is over. I do appreciate that DR was a powerful coping technique, reducing the impact of insanity imposed.

Anyway, hope this helps.

I can't help you with any system problems, but I do experience derealization a lot. Sort of like Borg, I mostly accept that life can be as real as it can be fake. Mostly, reading/watching something scary fixes it. It makes me worry more about the scary stuff than whether life is real or not. Talking to people makes it way worse for me, especially if I can see their reply coming. Sometimes sleep fixes it and sometimes not. I usually just tell myself that I'm already here, might as well live it to the fullest. And I also stay away from scientific theories about reality, even though I love thinking about the paradoxes. Hope this helps and good luck.