The Wildfire can be rooted. Just google for it. On the other hand - I would wait for the official release, and I'm not going to root it now and may be never. The Sense layout is rather nice in my opinion, but you have the Desire, so you should now.
I've seen the X10 Mini Pro, and it doesn't compare at all to the Wildfire. It's a different kind of device. The Desire and Wildfire are similar in many respects. The big difference for your dad (and I suppose he's not a power user) is screen quality. Speed will not be a big issue. Size and price may be.
I've tried the X10 in the shop, and the screen has the same resolution, but the screen is much smaller. With the hardware keyboard it's much more a texting device in my opinion. But I may be wrong. The WF is basic with a 320x240 screen, but it's good enough for me, considering the price.