I plan to have a "Dutch" style planted 75g. I'm going with the FilStar series, and I was going to use the XP2 for mechanical filteration and as my reactor. And then have a XP3 as my main filteration. Is this a good idea, or should I just use the XP3?

It's a good idea. It will give you enough water movement to avoid dead spots in the tank. In a fish only tank you would be able to get buy with just the XP-3. But due to the wood and plants in our tanks it's very hard to get that movement with just the XP-3.

But I really have to question your ideas here. In a planted tank you basically only need mechanical filtration. The plants should and will provide the biological filter. What kind of "main" filtration are you talking about?

I mean, it will be where I will use all the media options I have. For example, after an outbreak of Ick, I will add the carbon to the XP3, rather than the XP2. As for the biological filtiration I think I'm just going to use lava rock?