When it comes to selecting a track lighting fixture it's imperative that you select heads, or pendants, that fit your existing track system. Improperly installed fixtures can be dangerous, to both you and your home.
With that said installing a proper fixture is quite easy. Most simply lock into place while others need to be installed with set screws. Either way they install rather easily. Just be sure to read the instructions that come with both the track and the new fixture, to keep yourself and your home safe.
But how does one know what fixtures fit with what track? Well, there is a great resource from www.brilliantlighting.com using their custom design wizard. I went through this wizard and it seems to be very good. I have no affiliation with this company, nor have I dealt with them, but this seems to be a good program. Try it out and see if it works for you.
Now that that's out of the way, selecting a fixture is not only based on your existing track system but also your taste and style preference. When selecting a fixture it should fit into your existing kitchen, or your future kitchen design. That's not to say you shouldn't have what you like, but most times a fixture that goes with the existing style looks much nicer, in my opinion of course.
Here are some ideas:
Use a pendant over the island or breakfast area.
Use spotlights over a sink or work area for pinpoint light.
Use globes for general lighting to bathe the area in light.
Use a track system that fits your overall kitchen style.
These are some ideas for what fixtures to use in which areas but feel free to mix and match placements, only you know what will be best for you.
Now let's talk about track styles. There are three major track styles: Monorail, Cable track lighting, and the original track system. Most of the track systems are of standard sizing from company to company, but not always, so be sure to check your documentation before you decide to buy a specific design. Also keep in mind that there are line voltage tracks and low voltage tracks.
If in doubt of any of this consult an electrician, because installing the wrong head on the wrong track can have disastrous results. Again, there are designed safety features to prevent this occurrence but that's not say it's always like this. I don't say this to scare you, I simply want you to stay safe and not harm yourself, your family, or your house.
In closing, changing out a track lighting fixture, in most cases, is very simple. Check out the installation instructions that came with your track system, and you should be on your way to a new lighting design in no time.