I would like to use Zebble only for producing UI and all other things I would like to use Xamarin apis/custom http apis/local db or whatever it may be. Basically a UI project with zebble and other things will be in PCLs.
Will it be compatible? Could you please advice?
Yes you can. There is nothing in Zebble that prevents you from using the native APIs directly.
For custom http calls, I recommend using the HttpClient class which is by default available in all 3 platforms of a newly created Zebble project.
For Device APIs, you can of course use the standard API classes of each platform, but to save time and achieve 100% code reuse I strongly recommend using the http://zebble.net/docs/device-api. For example if you want o use the lamp (aka flash, led or torch) you can very easily implement that on all platforms with very little code code:
// Determines if a lamp feature is available on the device.
if (await Device.Torch.IsAvailable()) { ... }
// This will switch the lamp on.
await Device.Torch.TurnOn();
// This will switch the lamp off.
await Device.Torch.TurnOff();