I 'm flattered you asked, FireSoul!
To tel l you the truth, I'm not 100% sure on this one.
Being this is a modelers world (and I am certainly not one, just an admirer and collector), some may feel protective of their works. In this case, their websites usually have a statement regarind the usage of their models, posting pics, modifying them, etc. And if memory serves,
www.feralyards.co.uk does not contain any such statement. With that in mind, it could be assumed Thu11s does not feel particularly protective of his models.
And since there are no financial factors involved or copyrights, etc. the only real concern is artistic ethics.
To sum this up: as long as you make sure to give credit to the original artist and you have "modified" a previous work (which you have done), I see no real probelm EXCEPT perhaps Thu11s might be miffed. But in his case, I think he would be rather flattered. Maybe he would even post them on his own site (if you ever get hold of him that is).
Well, that's my two cents.