Unless its a barrage weapon its treated as any other weapon firing. The only exception is if you fire ordnance you cannot fire anything else. Also note that some ordnance barrage weapons can also be fired as regular ordnance weapons and thus can move and shoot if being fired in this way (basilisk, manticore, etc.)
The combat speed thing is not always true as a fast vehicle with an ordnance weapon (Blood Angel vindicator for example) can move cruising speed and fire ordnance.
A big portion of the Imperial Guard vehicle rules are avaliable for free totally legit. Go to the Forge-World site and find the updated Imperial Armour Volume 1 amendments to bring it in line with 5th ed. They list pretty much all the guard vehicles rules (medusa bastion breacher shells, manticores, hydras, valks, lumbering behemoth, etc.)