FZ1000's hot shoe; can I use any flashes? Or can I only use Panasonic flashes? Give me some details about the hot shoe...
The FZ1000 tolerates no more than 24V on the trigger contact (with positive on center). Many vintage flashes may deliver 300V or more.
For TTL operation use one of the following: - Olympus FL36 or FL50 (that can be found used on ebay) - Nissin I40 - Metz 44AF (or more powerfull, see compatibility list on Metz website)
Meike is known not to work properly with fz200 and fz1000, as these camera support integration method only and not pilot flash exposure evaluation.
On any iso-compatible flash or camera hotshoe, the sync signal--the one that fires the flash in sync with the shutter opening on the camera--is communicated by the pin in the center of the "square" of the hotshoe/foot.
So, to fire a flash correctly, you can use any ISO-compatible flash. It just has to have that square layout, use the rails as ground, and the contact/pin in the center for sync. This would include most flashes, aside from older Sony/Minolta flashes that use a different physical configuration. With a vintage flash, however, you may have to make sure the sync voltage is within tolerance.
But if you want more flash features than just firing in sync, then you have to get a flash that works for four-thirds or micro four-thirds (same hotshoe). The OEM brands are Olympus and Panasonic, but a small number of third party flashes from Nissin, Meike, and Metz are also four-thirds compatible. Features like TTL (automated power-setting based on metering), high-speed sync, or remote optical triggering, require a four-thirds compatible flash.
There are lots of flash choices for your Panasonic Lumix DMC-FZ1000 Digital Camera.
If you want TTL auto flash you must use an Olympus or Panasonic compatible flash that will communicate the settings to your camera.
For manual flash you can use almost any brand of flash.
Here is a link to B&H Photo showing many different Olympus/Panasonic TTL compatible flash units you could buy for your camera: B&H Photo Olympus/Panasonic compatible flash units
As a Panasonic camera, the Lumix FZ1000 accepts (Micro) Four Thirds System flash units. This means you'll get TTL functionality with any modern Panasonic or Olympus flash, as well as with any third-party flash designed for Four Thirds.
If you're looking to use a non-TTL flash unit, page 320 of the advanced manual states that the camera cannot accept high trigger voltages. If you have an old high-voltage flashgun, you'll need something like a Wein Safe Sync to prevent damage to the camera.