I have a Platignum No.5/Studio and I think it's a great pen, however, Platignum themselves and their various stockists all list the pens as taking Platignum cartridges only, and they don't make a compatible converter. I know a lot of people using this site have had success fitting international standard converters (the most common sized converter) and the one that most frequently comes up as fitting Platignums is the Monteverde branded one. However, a couple of months back, I tried fitting (well, "pushing quite hard" might be the closer to the mark, since it wouldn't go) an international standard cartridge to my pen when I ran out of Platignum cartridges. Not only would it not fit, but I seem to have cracked the nipple at the end of the feed as the next time I tried to do anything with it, it just fell off, rendering the pen useless until I bought a replacement (which I did, the pen is that good). If you are buying this pen I'd stick to the cartridges, (I do, now) - they can be difficult to find, though and if it's only the pen you are chary about buying online, you could still get the Voyager in the shop and then buy your cartridges online, through a specialist pen site (if you let us know which country you are in someone on FPN will be able to help you on where to get cartridges online without paying massive delivery charges).
To answer your specific questions, they only come in a medium nib, but that is slightly on the finer than average (if it's the same nib as mine).
As I said, Platignum are manufactured to take Platignum-only cartridges and they don't do a converter.
I've had no problems (apart from the one I caused myself) with my No.5 Studio, but I don't have a Voyager.
Honestly, they are great pens, but probably not a good option for someone who isn't keen on using the internet for buying things. As top pen said, of the three you mentioned I'd probably opt for the Vector - it's tough, reliable, a very smooth writer and you can easily get both cartridges and converters just about anywhere. If you can (and you haven't got your heart set on pink), I'd get one of the all-metal ones; they're great value for money and nearly indestructible, in my experience. The 15/Jotter uses the same nib as the Vector (I'm surprised it's sold as a 15, I thought they were renamed Jotter back in the 1990s) and is even cheaper if you are just wanting to try out a fountain pen, but it's also even smaller, which means that unless you have really tiny hands it probably isn't a good option (I own both a Vector and a Jotter and, in my opinion, the Vector is better than the Jotter, but neither are bad pens).
I don't want to throw cold water about the idea of getting a Platignum, they are great pens and excellent value for money, except for the proprietary-cartridge-only thing. I really think Platignum are doing themselves a big disservice. I'm still trying to find out if there is a 100% reliable way around this without breaking another pen, but I haven't got a definitive answer yet.