Well, for me, I can't see most of TSL's alpha channeled textures very well, (Dammit Intel...) so no alpha means no semi-invisible HKs. But I hope you figure it out.
Also I just realized, the way HK-51 looks in SWTOR is kind of like a mix between HK-47 and HK-50. HK-51 features a grey paint-job with orange highlights (so it's like a mix of his predecessors), and has an eye color that looks in between 47's Orange eyes and 50's Yellow eyes... maybe Amber or something. I guess it's supposed to signify 51 as being the superior model, being a mix of the two.