Yep Onslaught, and e.g. Vanguard DiD, GZG, and most others work fine with the Epic. I have pretty much have all of them.
Now now note GW Epic had a scale creep situation, over the years even way before the new 8mm Titans.
E.g. Mk.VI SM is a little smaller than the Mk.VII. SM Scouts and newer leaders/characters[i.e. Medic, Tech, etc.] are bigger than both.
Eldar come in 3 sizes as do Orks and even Chaos SMs … sizes grew as time went on … With the different Epic releases, i.e. AT1/SM1, SM2/TL, E40K, and finally E:A[It uses mostly the larger models from SM1/SM2, & E40K].
But I don't have a problem with it, it is what it is and GW is what it is … Been doing Epic since '90. For better or worse …