[kramarczyk]

Before anyone can really make helpful suggestions we need to know what is your goal for this chassis. Additionally, where were you planning to use the two types of wheels? My preference would be traction on the center and one end with a pair of omni on the other end, but there are other configurations that folks run (omni on all corners, etc). Finally, what are you planning to drive on (this is related to the first question)?

Absent all that info, my best suggestion would be to assemble it as you planned, try it out and see what you notice before modifying anything.