Sparhawk:

That's Johnson and I doubt the UNSC at that time would know enough about the covenant yet to pilot a scarab.

Sparhawk:

I doubt he could do it then, remember he achieved that battle in Halo 2 some years after when this game takes place, I don't think any normal soldiers, Spartans alike would know how to pilot it... not when they know so little about the covenant at this time.

You can't expect ODSTs and conventional marines to have the ability to capture and pilot the scarab at this time, plus Johnson won't be in the game-play nor anyone with his capabilities excluding the Spartans.

This is where i disagree. Like i said above, its probably not nearly as difficult as you make it out to be. I bet the only reason more humans don't know how to use any of this stuff is that its probably tough to come by and so materials haven't been distributed so much. 

 an engineer class would probably be best for this.  Besides, i'd hate wasting ODST or spartans as pilots.  I'd rather them just be able to knock the walker down and then have an engineer or someone take care of it (engineers having advanced knowledge and capability).  That way it fits.