The Jackson Mystique is a MUCH lower level boot than the used ones you have, that's why they feel much softer. I'm not familiar with the brand of the used boots, but physically they look okay, and were probably originally a low-mid level freestyle boot (which should be okay for you since they are used), and the blades look to be in good shape as well - there is plenty of sharpening left on them and I don't see any rust, but since it's not possible to see the rocker profile well from the images, I second the suggestion to visit a sharpener just to make sure they're in good shape to skate on (as in, sharp enough, no previous sharpening damage done to the profile of the blade), but I don't see any reason why you shouldn't be able to use them.
If you have pain over your ankle bones because the leather is stiff, you can have the boots punched out in those places (which is very common, most pro shops can do this for you for a small fee), or try wearing gel sleeves (either by Bunga or Silipos). Silipos are usually cheaper, you can buy the 10" tube for the price of one 5" bunga sleeve and cut it in half so you have a 5" sleeve for each ankle. I just bought one on Amazon for $16.99.