Can you have different styles or should you stick with one? I feel like I have contradictory styles. I love tattoos, and heavy metal, and motorcycles, but I also like suits, ties and a expensive watches.
How can I best combine both without being thought as a poser?
The one you feels best. No matter what you are going to wear or put on or use, think about the combation which makes you feel best. Have you ever considered about some Tahitian pearls? Ther are pretty cool.
I think anyone that thinks your a "poser" is probably too immature anyway. A lot of people like different styles...it's no one's business to say what they can and can't wear.
To many people these days are concerned with fitting into a specific mold. Just be you man, honestly. People will respect you for being well rounded and I find that girls tend to like a guy that can fit into different molds. Some days I wear my cowboy hat and wranglers, some days I wear my Yamaha racing shirts and jump on the motorcycle and if its a nice night to go out I have a couple suits or button down shirts with a vest. It's not so much about what you have on, but the confidence you carry while in multiple situations.
The key is to meld both styles to create your own.
I have a weird nerd+classy+tough guy look going on for me. I love it and the ladies love it. For example, a typical day I'll wear my Lucky Brand jeans (they're all I wear, only ones comfortable enough for me), a Skagen watch, a tight-ish (don't get too crazy here, just enough to show off some muscle definition) old school Star Wars t-shirt, ray bans if it's bright enough, and a blazer if it's cold enough. And I almost always wear my chucks, but I'll mix in my Adidas every now and then as well.