Are there any disadvantages to lowering the bike?
How do I lower the seat height/get my feet more firmly on the ground?
You can lower the bike by installing adjustable links to your bike's swingarm. The suspension links are from Adjustment Tech Racing and cost about $250. This is not really advised, due to the fact that it can, and usually does, cause bad handling that will be WAY worse than having to get comfortable on tip-toes when stopped.
Things that should be tried first are:
Once you are comfortable with the bike, you don't need both feet firmly on the ground at stops anyway (just one).
Other suggestions for short people.
The lowering kit disrupts the fine handling and at best makes the 250 act like a cruiser (it lowers the back and extends the front some). This is well known among racers and is why novice racers will tend to run wide on corners if their rear shock isn't up to the task. At worst it will make the bike Extremely twitchy at speed and have drastically less front tire traction. Changing the geometry of the bike is something that should only be done after consulting with professionals... and there's not a suspension company in the world that'll lower the back with just dogbones and not modify the shock to still work correctly. Most often the front forks will need to be lowered, to try and keep some semblance of the correct geometry. It's just a lot more complicated, with much worse consequences, than most people think.
Use the BrianM litmus test: Would you want your significant other riding a bike that didn't handle as well as it should? The best alternatives, after the "things to try first" mods described above, are:
How to tell if your dogbones have been altered
If your dogbones are stock, they will look like this one and have these measurements. The aftermarket/lowering versions all look different than the stock one.