I would need more information on what you mean by forms and what you are try to accomplish. If it is how to fight with a knife/Tanto, there are many good styles. I am partial to some of the Israeli commando forms.
If you are looking for defense I can comment on this a little. I have trained in Aikido for many years in the US, Canada and Japan. When I asked the similar question to our master and sempai in Japan, most do not train with the weapons. They admit it is useful, but it is use to present a priority when doing a technique.
They all said basically the same thing. Practice your basic movements without a weapon. Make sure you are clear and efficient. When you do add a weapon, you will have to move a little sooner and be clear in taking of balance, pins and throws.
Ask yourself when doing a technique, am I in a position to strike with a tanto or atemi?
This same advice was given to me when I studied American Kempo and Tae kwon Do.
For me we introduce Tanto at Nidan, but it is still just Yokomen Uchi Shihonage or tsuki Kotegashi, the only difference is the attacker has a weapon. Same at Sandan we have a Jo and Bokken.
So in the end, my recommendation is to be proficient in your style, look for openings to strike. In the end no one wins in a knife fight. Someone always gets cut.