L0d3x:Cyclops, flamethrower and Prophets on the battlefield.
I'll explain exactly why the last two are just not right, im not very knowledgeable about the cyclops however.
But I have class now, I'll update this post late.
Ok first off I will give you the generic version of why this is wrong.
What sources do you have for canon during this time period? All we have is the book Halo: Contact Harvest and that bairly scratches the surface of the war. No in fact it bairly scratches the very beginning of the war. We have no data on units, tactics or any other issues during that time period other then dates of fallen colonies.
This is a huge issue as we dont have this information. Therefore any argument about canon being wrong for this game is completely ludicrous. You have no canon to base your arguement upon. They are dealing with a time period that none of us has seen before. Its a completely new set of canon.
Now the spesific versions.
Cyclops:
Once again we dont know what kind of special weapons projects or existing units were around during that spesific time period. There have been several plausable theories as to how this unit fits into this game. I wont go into those details though. You dont have to like it but it is completely canon at this point. There is no evidence that this unit was never in the Halo Universe and thats final. I challenge you to find me something that says other wise.
Flamethrower Marine:
The flamethrower has appeared in several Halo games already. Why wouldnt it already be around during this time period? I specifically point you to the earliest sighting of the flamethrower. That would be in Halo: Combat Evolved. Its in the PC version for multiplayer. The it again appears in Halo 3. Its not pretty but its effective and very very useful.
Prophets:
Prophets are widely known to travel along side their fleets on flagships. It is very possable that during an engagement that some prophets were forced to be on the ground during combat operations. Granted they would avoid combat at all costs it still is possable that they could have been forced in to that roll at one time or another. We just dont know enough about the other battles to truely know this for sure. So again there is no actual canon saying otherwise.
In fact if you look at the instance at Instillation 04 you will find a Prophet in an actual combat zone. This can more or less support this theory.

Approval of situation A recognized. Commensing the Cromwell Invocation. Ability restrictions lifted for limited use until the enemy has been rendered silent.