JuliusCaeser,
If an airline is asking for 300/500 hrs on type, what they are effectively saying is that they do not wish / do not have the capacity to take on the training burden of completing initial type line training.
This is why having a TR but with no hours on type is rarely an advantage. As a previous trainer, if anyone failed to make it through training it was usually because they couldn't complete the line training to standard in a satisfactory time scale. By specifying at least 300/500 hrs on type it shows the airline that you are of a standard to pass the line training of your previous company and therefore reduces the risk that you are not up to standard - (but in experience, this doesn't always follow!)
It is often preferred for an airline to bond you / make you pay for TR internally (such as Ryanair) and have you flying with their SOPs / philisophy from day 1 of the TR.
To summarise: I would strongly recommend against getting a TR for either of those types of your own back. In my career I have come across a good dozen or so colleagues in the RHS that attempted to get their first job by doing just that. I can only think of one person where that worked out for them.