Here are some suggestions on ND use. You are using a 6D, so I would start with a base iso of 50 or 100. With this, you should be able to get to 1 sec in normal outdoor light with a 0.9 (3 stop ND filter) combined with a CL-PL. I recommend the CL-PL around water to cut the glare. It makes a big difference. Most CL-PL filters are between 1 and 2 stops when polarized so with a 0.9 ND you will have around 5 stops. I like to shoot water (for the blur effect) between 1/8th of a sec and 5 to 10 seconds. If it's early or late in the afternoon I might push to 20 seconds. The 16x and 32x ND's from Hoya will get you much longer times but for the effect of water blur in normal streams, they are a bit too much. They are great on ocean shots or sky (cloud blur). Lee also has a filter called the big stopper which works in much the same manner.
You mentioned having a Samyang 14mm and the Zeiss 21mm. The Samyang will not really work with any filter solution I know off, even Lee setup. Lee makes a great solution for the Nikon 14-24 but I don't think it will work on the Samyang. You can't really hold a large sheet filter in front of the Samyang as the tulip sun shade will let in light on the cutouts. Lee makes a 100mm step down adapter, meant to work on a 95mm outer diameter. I am not sure what the outer diameter of the Samyang sun shade is but if it's close to 95mm you might get this to work. You also may be able to adapter a Cokin pro holder or High tech.
The Zeiss will be a lot easier, you can pick from either a few 82mm ND's or get the Lee holder (foundation kit and 82mm wide angle adapter ring). This will let you use (2) 4 x 4 Lee filters. Lee makes two glass 4 x 4 ND's 0.6 and 0.9 both are excellent filters, but a bit on the expensive side. They also make a 4 x 4 square CL-PL that will fit the holder. As long as you use a solid ND Lee or other brand you can use the CL-PL by Lee. A grad will be messed up when you rotate the CL-PL. You can also pick the Lee Wide Angle Hoood, with 1 slot or 2 slots and it will work the same as the foundation kit, but also has the hood. This is what I use on the 18mm Zeiss. One note, if you pick a Lee solution you will need a wide angle adapter ring. You can't use this ring with a normal 82mm CL-PL mounted first, the rotation feature of the CL-PL will make it so you can't get the filter off. The Lee ring fits over the outside to help cut down on vignetting issues.
After carrying a bunch of different ND's and CL-PL's with step up/down rings for years, I have moved to Lee for most of my ND/CL-PL solution. The ND's that are glass are excellent and I can't see any difference in image quality in the Lee vs. a Hoya or Tiffen. Lee claims a bit of IR blocking, but I am not too sure on that. The longer you go, the more IR pollution will possibly effect the outcome. Tiffen makes a great line of IR-ND's but again they get expensive.
Grad ND's are a different issue. They work well on sunrise and sunsets, but if you have any foreground object, trees, rocks etc. that are in the area of the ND, they will be adversely darkened and it will look a bit un-natural. Grad ND's work great on open horizon lines. You can also consider the reverse grads by Singh-ray. Darker near the center and lighter to the top, and on a sundown, sunrise they put the darker area near the center where most likely the horizon will be.
For most sunset, sunrise work, I would instead consider exposure bracketing. The 6D will allow you up to 5 brackets and as much as +2, -2 per bracket (I may have this wrong). Excellent solution and there are many great software products to assist in the creation of the image. Nik HDR 2.0 is a great tool for this as it allows for very nice blends without the hard edged 3D look.
If you are in the U.S. I would start at www.2filter.com
, ask for Andrea. Great company, and they carry a wide range of brands. They also have IMO the best connection with Lee in the U.S.