Any dose above the prescribed amount for the particular condition an individual is being treated for is considered an overdose. So any dose above what is listed on the prescription is an overdose.
The effects of taking too much of this drug will vary depending on factors such as the status of a particular individual's kidneys and liver (renal and hepatic function) and whether or not they have any tolerance towards the medication (tolerance can develop when the medication is taken over time). Additionally taking clomethiazole along with other medications that potentiate it's effects (such as the sedatives you mention above and/or alcohol) will decrease the amount needed to produce an overdose.
To be clear, yes, zolpidem, lorazepam, and valium all interact with clomethiazole, potentiating the sedating effects, thus decreasing the amount needed to overdose.
There are many additional medications that inhibit metabolism of clomethizole, meaning that the effects of the drug will be increased, resulting in overdose, for example, cimetidine, calcium channel blockers, diuretics, antibacterials (among others). This is why it is important to tell any doctor prescribing medication to you any other medications that you are taking.
An overdose is considered any dose higher than what is prescribed to the individual for the condition they are taking the medication for. This medication is prescribed for insomnia, withdrawal syndrome, status epilepticus, and pre-eclamptic toxemia.
An overdose of this medication can cause respiratory depression, meaning an individual taking an overdose may fall asleep and breath shallowly. The lack of adequate deep breathing can result in anoxia (lack of oxygen) to the brain, and permanent brain damage may result, even as the patient recovers physically as the medication is metabolized by the body.
Please reply if you have further questions about this topic.