The Nissin works okay, but its only real selling point is it supports TTL metering, which no other flash currently does with Fuji. However, it's also very small, the light quality from it isn't terribly great, and there's limited control. It's a big step-up from the little flash you get for the X-T1, but if you're trying to equal out midday sun, forget it.
A larger, simple flash unit like Neewer or Yongnuo flashes will work for this purpose better, as they are simply bigger and have more light output, but they only have basic manual control. Cactus flashes give even more light output and have lots of fancier features—remote zoom control, customisable delay timer, an update soon for HSS, etc—but they're still a manual control flash, on Fuji.
Anyway, bearing in mind that ND filters are a bit irrelevant for portraits without flash...
What ND filter you use varies depending on the day, the size and proximity of the reflector, and what shutter and ISO you want to use. (And aperture, but you've already said you're looking at f/2.8.) I carry 1-stop, 2-stop, and 4-stop ND filters with me. That way I can cut any amount of light from 1 to 7 stops by adding the filters together. I use Hoya Pro ND filters. They've no noticable colour cast and are even right across the frame. They do reflect the inside of the lens barrel, though, so a a tip is to draw over the white writing on your lens with a black pen. (Look on the front of the lens, you'll see what I mean.)
On an average bright day, I'll mostly use 3- or 4-stops of ND to get the light low enough to use something like f/2.8 with ISO 200, 1/125th*, a white reflector, and a Cactus flash on quarter power. On really bright days with absolutely direct sunlight on the subject, or reflected sunlight on them with a silver reflector, I'll use 5-stops; 6-stops would give a 'proper' exposure, but I like my bright sunny pictures to look bright and sunny, so I let them over expose very slightly.
There's no one single ND filter I'd recommend to start out with. It's really best to get at least two. A 2-stop and 3- or 4-stop would be a good pair. Then add a 1-stop once you're used to working with combining filters, reflected light, and flash. 'Course nothing stopping you from buying all three at once, if you can afford it. Point is, there's no one grade of ND which will definitely be right, and variable ND filters should definitely be avoided.
*I use 1/125th because that's pretty much the minimum sync speed that all cameras and lights can use, from the cheapest and oldest small units to big modern studio power packs. By sticking to 1/125th I can forget about matching sync speed when I use different systems, and it's just one less thing to think about. This does require a little more ND for flash in strong direct/reflected daylight, though. You could use a stop less of ND and just increase the shutter to 1/180th, the X-T1's maximum, and accept a tiny bit of over exposure, or you could go up to 1/250th. Fuji don't officially support 1/250th on the X-T1, but some people report using it without problems.
