If your inquiry is may the Sirui ET-2004 Aluminum Tripod with E-20 Ball Head be used with a gimbal head and your Nikon D7200/Sigma 150-600mm lens combination, then the answer is yes. The combined weight of the Nikon D7200 DSLR camera and the Sigma 150-600mm f/5-6.3 DG OS HSM Sports Lens for Nikon is 7.79 lbs (3.53 kg), and the Sirui ET-2004 Aluminum Tripod with E-20 Ball Head has a maximum load capacity of 26.5 lb (12 kg).

The load capacity will be sufficient for your gear.

Yes, the built in plate on the tripod collar would mount directly to the ball head.

The TY-50E Quick Release Plate included with the E-20 Ball Head on the Sirui ET-2004 Aluminum Tripod uses the standard 1/4"-20 male tripod screw. If your camera has a 3/8"-16 female tripod socket, you would need a reducer bushing such as the Oben 3/8"-16 to 1/4"-20 Reducer Bushing, B&H # OB1039, or you can purchase an Arca-type quick release plate that includes a 3/8”-16 screw, such as the Acratech Arca-Type Universal Quick Release Plate (3/8"-16), B&H # ACQRPCT38, the Really Right Stuff B29 Multi-Use Fore-Aft Plate with 3/8"-16 Screw, B&H # REB29PLATE, or the Sirui TY-70A Quick Release Plate, B&H # SISUTY70A, for your usage needs.

Yes, there is one spirit level on the ballhead.

Can't speak to that specific model but the lbs rating lives up to whatever you can throw at it. This tripod has heavy duty construction amd is very stable. You'll be hard pressed to find a better value or a more compact tripod with this weight rating. I had to tighten down the leg supports after a couple uses with heavy gear but i haven't had slipage since. Again, you won't go wrong here.

I don't think it will convert to a monopod.A long lens could work on a 200 to 500 depending on where the mount bracket is. I have a Tokina 150- 500 that I use on a monopod most of the time for sports action it has the mount in the middle and is well balanced

The E-20 Ball Head will allow the camera to aim straight down along the center column of the tripod.