all of those models are much too small for it. its supposed to be on a 100x150 base. do a search there has been many discussions on converting a destroyer.
i have been thinking from the following
a balrog
a bloodthirster
a cygor
i have also seen a really greay looking hordes minotour that looks very good
richard barby
Of those I think the Balrog would be the best pick.
Yes, it's a bit small for the base, but thanks to it's pose looks alright. And it got the right look ;)
Not sure how to make a firedaemon out of a cygor... I'd love to see that hordes mino, you mentioned...
So here're some usual models used:
FW Bale Taurus w/o wings (probably the easiest way and good looking)
GW Ogres Stonehorn (this is the one I'll use. Needs some conversion and VERY dependant on the paintjob... I'm still not happy with my conversion, as I'm unsure wether to use the stonehorn's head or a gorghon one)
Defiler / Soulgrinder (Machanical Approach, not my taste, but gets big enough ;) )
Random Plastic Robot Toy (Iron Man f.ex.)
Now I can only repeat, what DA Crusha said: Have a look arount the forum (especially the conversions area) there are many ideas floating around.