The Space Marines and most MEQ armies have a difficult time at 500, no doubt. I've written up some SM lists in this thread, and they're okay, but they don't bring all the tools I would need.
My first 500 SM lists looked like yours, but I realized Scouts are too inconvenient as my only Troops choice. If the Scouts move out of terrain, they're essentially dead. The Librarian didn't quite fit in anywhere. And mathhammer suggested against the threats you've rightly assessed, I'd probably be off the table faster than I care to be.
I think the front AV 13 of Predators could hold up nicely, though. If you end up taking a Tac squad, you'd only be able to afford one of them.
So here's another list.
Librarian, Null Zone & Might of the Ancients
Tac x5, Sarge w/ combi-melta & power fist, Rhino
Tac x5, Sarge w/ combi-melta, power sword & meltabombs, Rhino
Scouts x5, Sniper rifles x4, Missile Launcher
And if you like the Predators more, take out a Tac squad for 2 or the Scout squad for 1.