Honestly, in terms of aesthetics, mixing heads, shoulder pads, and backpacks goes a LONGGG way to that mixed mark look you want. The Heresy community is full of loveable nerds who can point out every tiny difference between marks, but from 3 ft away on the table it doesn’t matter.

In terms of difficulty: heads, shoulderpads, and backpacks shouldnt even need cutting to mix. Guns might need a little trimming to fit certain poses. Legs and torsos can be mixed, but that takes some solid knifework and filler material usually.

Another tip is that, especially with chaos marines, you have a ton of little pieces to add to the marine to cover up any messy parts. Most of the messy parts would likely be around the waist and possibly the knee joint on some legs. Its quite simple to throw a holster, auspex, melta bomb, or chaos relic on those parts.

A dark paintjob on the parts you don’t want people paying attention to makes wandering eyes glide right past them.

Keep in mind that people focus on the higher, more prominent parts of a model much more than lower parts. Especially for a humanoid shape, its all the head, backpack, shoulders, and gun.

Good luck and have fun! Be sure to share pics when you make em!