IMO, you'd be better off using glass fuses.
Polyfuses are best for situations where you need current limiting, but it's hard to get into the circuit to replace an actual fuse. Their behavior is also more complicated, since their trip current (where they're guaranteed to shut off) tends to be much higher than their holding current (the amount that will pass through normally). If you want the trip current to be 5A, you'll only be able to use 2-3A as holding current. If you want 5A of holding current, the trip current will be up around 10A.
Your cheapest and easiest option will probably be automotive fuses. Those are made to work with 12v supplies, can be found almost anywhere, and are reasonably inexpensive. A good auto supply store will probably have best range of fuses to choose from, and will also have holders that you can hook into your circuit.
You might also look at putting fuses on each of the lines supplying power to your servos. That will keep a single overload from killing the whole circuit, and will give you a better idea of where the problem occured.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.