The video reminds me of something my dad told me long ago "Do it right or don't do it at all" I haven't thought about that in more than 30 years. If I had to teach a ninjutsu class. I would follow it the same way that I follow my Jow Ga Kung Fu and that is.. "We don't play on the ground, we escape from it." I would stay true to this as much as possible and just focus on being good with 2 things in terms of ground fighting.
1. Escaping and countering any of the opponents attempts to be put on the ground.
2. Escaping from the ground once the opponent is successful with #1.
3. If I go to the ground it should be of my own choice and not because of my opponent's successful technique. Being on the ground should always be for a moment.
If I can be really good with #1 and #2 then I can deny the ground fight and deny / avoid where my my opponent is stronger than me. I wouldn't add anything to the system unless it works well with #1 , #2, and #3. My opinion is that once Ninjutsu gets into having 3 minute grappling matches, then it has moved very far away from the concept of finishing the fight quickly and as a result it throws away a lot of the concepts and true focus of what Ninjutsu was. Now if there's a BJJ technique that will allow me to stay within 1-3 then go for it.
Yeah, now you're entering silly territory.
There are people out there that will put you on the ground, and keep you on the ground. If MMAists, Judoka, Jiujiteiros, Samboists, and state and collegiate wrestlers can be taken down, you can be taken down as well. Unless Jow Ga has some grappling escapes I've never seen before, punching, kicking, and clawing isn't going to cut the mustard. Once the first blows start dropping from the superior position, you're going to start reacting instead of thinking and it'll be all over before you know it because you're going to be taking very significant damage.
Additionally, it shouldn't take me, or any mid-to advanced level grappler 3 minutes to subdue someone on the ground unless my opponent is also a mid-to advanced level grappler. If you have no grappling experience and a grappler is on top of you, you're screwed unless you've trained against it.
Don't fall into the trap of thinking that whatever kung fu skills you've developed will protect you from gravity, they won't. This is especially true in the US where grappling is exploding in popularity and people are flooding into wrestling, BJJ, Sambo, and MMA prgrams and mixing everything together to create even more potent grappling systems. Frankly traditional arts aren't keeping up with that evolution, much less learning how to properly counter it. Some are adopting Bjj into their programs, which is a good thing, but unless they're doing dedicated programs, it may not be enough.
In short, there's nothing wrong with some cross-training. I'm sure you can find some Bjj or Wrestling in your neck of the woods.