Here is my opinion. First I must add that I have studied electrical engineering for 3.5 years at Clark college and Washington state university with success.
When ever you add an electrical component that has any qualitative amperage draw, you create an instability in the hertz (waves) of how the voltage flows. Theoretically you could add any component to the bike, even with a small 7L battery but you will need to stabilize the current flow.
To stabilize the flow for an HID kit as an example, you need to ensure that the wires are gauged thick enough to handle the power consumed. Then the positive wit supplying the energy to the system needs to have a couple of small resistors to stabilize the circulating flow of electricity. You also need to make sure that you have small capacitors next to the final destination of the power consumption. And finally you would be wise to add a compact 1farad capacitor from the battery to the headlight wires.
With that as an example of how to properly install an HID kit, you can see that some work will be required if you decide to add an aux power supply for gps/etc, but then you would need to worry about water. Rain gets on the contacts and you will definitely have a short so be sure to include a 20A circuit breaker or replaceable fuse.
Moral of the story, you CAN do it, just research how to do it the RIGHT way, vs the fast/cheap way.
Yes a basic install will have your components working for now, but the redundancy I planned in my earlier message would prevent malfunctions in when there is a high power draw I.e. Lighting system or high out put motor upgrades that require more amps. If you are just charging your phone @<1.5a, you should be ok, but I would just upgrade to an AGM battery at that point to prevent irregularities. Maybe I just over build what I make, but it never hurts to have a little extra cushion. Best of luck with the install!!
If you want to install anything for navigation, you may as well just buy a water proof case for your phone, install a charger for it, a mount, and use an app as your gps. Knock out two birds with one stone. But if you are really set on using a dedicated gps, you can't go wrong with a Garmin.