I too have never been a rabid Titan fan, I have about 25 or so of all types including Gargants, etc. I bought SM1 before AT1. And as I said before, we use Titan sparingly. I have a Pycho-Borg from Alternate Armies 15mm range that I'm going to use as a Chaos Titan. And I was going to use the SC Dread as a Titan too, but traded it to a buddy. So I think you can easily use larger scale models with little or no modification as Titans. And I think there is a lot of stuff to choose from. Grendal has some 25mm Fantasy (!) pieces that would work too, IIRC. So I say go for it ! And as far as scale, G/W has always been off ! No way will 10 troops fit in a Rhino or Land Raider for that matter !
You'd be hard pressed to get one 6mm Guardsman in any Titan head ! But that's G/W ... and now F/W too ! 
But those Titan classes/varients were the original types. The Warounds had varients too (Mastiff and Wolf - issue 109 W/D), and you see that Forge World named their type varients also (Jackel and Wolf [again !]). The names/varients were based on weapons configurations, not necessarily by weapons type. W/D 109 also lists the Vandal varient of the Reaver. But many of these are standardized configurations, like Left arm and Right arm always have the same weapon ie. Las-cannon and Auto-cannon respectively, for example ... 