Hi,
Which platform is the train for Tulcea (I assume I must put the standard introduction to questions first) - yes, you must first add the "Scuzați-mă..." or "Nu vă supărați..." before the question. The pronunciation is in the above lesson.
"Which platform is the train for Tulcea?" is translated into Romanian as: "La ce linie trage trenul spre Tulcea?" . This is the most common way to ask it. It's quite unlikely that one might use the word "peron" in his/her answer, if they do it has the same meaning as "linie" which means "platform".

"Is this the bus to Bucuresti?" - "Ăsta e autobuzul spre București?" - this is informal, but it's the natural way of asking a question on the street in Romania.
"Is this the train for Sibiu?" - "Ăsta e trenul spre Sibiu?"
"Where do I catch the bus to Bucuresti?" - "I suppose you mean where is the bus stop for the bus that goes to Bucharest. Can you please confirm that this is what you mean? There is another word that you will hear most probably: "microbuz" which means mini-bus. For the bus tickets, if it's a privately owned company you buy the ticket in the bus. If it's a public bus you must buy it most of the times before you get on the bus.

Do tickets for Romanian trains need to be validated as in Italy? Would this also apply to prebooked tickets/reservations? - The tickets are validated in the train by the train controller. You just need to have it with you while being in the train. There is no need to validate the ticket before you get on the train. The train tickets are for a specific train for a specific hour and for a specific seat number. I have added above in the lesson a typical train ticket.

The validation is just punching a whole in the paper ticket. It applies as well to the prebooked tickets/reservations. As long as you have a particular date and time for the train, which as far as I know is always the case for Romanian trains (I'll double check to see if no new tickets have been release since I last traveled), you do not need to validate the train ticket before you get on the train.

Some of the journeys I have planned involve changing trains or changing from bus to train. What is a reasonable time to allow for connections?
- It depends. As expected trains usually arrive on time, while buses depend on the traffic. The best would be to take some contingency. It also depends on the distance you have to walk between the train stop and the bus stop. In 99% of the cases I would expect a bus stop next to the train station.
- The best would be to check your particular case. You gave the example of Tulcea, here is the website of the Tulcea public transport where you see all the routes and if you click on any stop along the route you will see the schedule for that bus stop. I believe that more and more transportation services have their timetables online. If you have a particular need just let me know and I'll look it up for you.

Generally speaking, there are quite a few other advises for traveling to Romania. I'll do my best to write a few of those on a dedicated page. I hope it helps