Linebacker is a good choice (as "second line"), as is the Goshawk which also has been mentioned, both seen in the Hellion touman. Second line 6/9 heavies is just something that's missing for the Hellions (and really the Clans at large). You can come up with reasons for this, or just leave those heavies out of your faster lances, after all the Hellions do use 5/8 mechs as well (such as assaults like the Gargoyle) and you'll find more mechs there.
You can also make your Mechs part of Delta Galaxy, which was headed by a former Ghost Bear who beefed up his force relying less on speed (although he's still a Hellion - less being a definitively relative term here).
I'd form two stars, with the Predator and Thresher forming a heavier lance moving 5/8 and 6/9 (nothing slower) and lighter mechs like the Icestorm, Hellion, and the Solitaire. With a large Hellion force myself, I'd be happy to recommend other mechs to fill it out (such as all Hellions! :D)
At the end of the day, try it out. Although you'll suffer from the different movement profiles, you'll also have a force that's fairly flexible as it possesses the Mechs to actively harass while also hitting with some punch - hammer and anvil of sorts. And having a couple of "slow" heavies can be really advantageous as there are times when you need to hold an area or even just need a safe zone for your faster mechs to retreat to and the Thresher and Predator can provide that.