I'm going to give you the simple answer first, then completely invalidate that simple answer in the following paragraphs. But as a rule of thumb, if you're exceeding these numbers, maybe reconsider your modelling techniques.
Fighters and Bombers: 5,000 to 10,000
Small Capships: 25,000 to 35,000
Large Capships: 80,000 to 100,000
So, short and simple answer: Your numbers are 100% fine and well within the range of acceptable (on the low side even).
Now, the actual, more complicated answer: It's not as simple the total number of polies. Several things are going to affect performance, not least the number of polies per Subobject, where I believe FSO still has some hard coded limit around 64k, though someone can correct me there. Number of textures also matters, you ideally want no more than one per subobject.
What really matters is the efficiency of your modelling. You can make a very high poly model with very little actual effective detail by modelling inefficiently, splitting up your faces where you don't need to, and putting too much detail in places it won't be seen. Look at my GTFr Atlantis for an example of very inefficient modelling: If I were making that today, I could cut the number of polies by half and not lose any noticeable detail whatsoever.
Essentially, what I'm saying is that target polycounts or maximum polycounts have to be thought of in the context of what you're trying to make. If you're building a Terran fighter, for example, with lots of flat planes and straight lines, maybe you only need 5,000 polies. But the same fighter for the Vasudans, with lots of curves and strange angles that demands a different modelling technique might be perfectly justifiable at two or three times that figure.
There are, obviously, limits to that. If you're making a 20m long fighter and you've hit 30 or 40 thousand polies, you've gone too far. You're going to be putting too much unnecessary strain on the rendering engine if you want to show multiples of these on screen. But there's a lot of flexibility there.
As for your second question, is it important to make a really detailed ship as a newbie: Absolutely not! In my opinion, here're the primary thing you should focus on as a newbie modder: Getting your ship in game.
I would estimate that roughly one in five models people show off on the forum or IRC or p3d or whatever actually make it in game, and that's probably generous. What tends to happen is that ships get modelled, and then people lose interest when it comes to the boring parts: UV mapping, making LoDs, debris, converting etc. All of those steps are made substantially simpler if you're working on a thousand poly model as compared to a 10,000 poly model, and no matter how pretty the 10,000 poly model is, the 1,000 poly model that is actually working in game is the better model if the alternative is just sitting on a hard drive somewhere waiting for a UV.
It's good to have a general concept figure in mind: there is such a thing as too few polies. If you've built a 2km long destroyer and only used 500 polies, then you are absolutely going to see huge improvements by increasing that to 2,500 polies, or 5,500 polies. But eventually - and fairly rapidly - you'll start to see diminishing returns. In short, when you're starting out making ships, model until you're reasonably happy, then stop. It won't be perfect. But it will probably be much more manageable in the remaining stages than if you keep going until it's perfect and has the "right" amount of polies.