For the grammar point ～ないことはない, it says “this expression may be used with the ない form of any verb, い-Adjective, or な-Adjective.”
However, one of the example sentences given is: ここは危ないことはないけど、気をつけた方ほうがいい。( It’s not that this place isn’t safe, but you should be careful.)
But I would think the ない form is 危なくない.
So i guess my question is, are adjectives like 少ない and 危ない special types of adjectives? Are there any other い-adjectives like them that end in ない?
In the example sentence the grammar is simply ことはない. To my knowledge, 少ない and 危ない aren’t special in any way. ことはない by itself simply means “…the concept of [adjective] does not exist” (see details below if you’re interested in how I get this), while ないことはない means “…the concept of [negated adjective] does not exist”. So what we are seeing here is better translated (imo, to maintain the original japanese) as “It’s not that this place is dangerous, but it’s best to be careful”.
In short, ことはない is, in a more literal sense, “As for the concept of […], it does not exist”
こと means “thing”, but to be more specific, it means “concept” or “abstract state”. For example, let’s look at ～たことがない (have never done…):
東京に行ったことがない literally means: “The concept of me having gone to Tokyo does not exist”, and in more natural English, “I have never been to Tokyo before”.
Now, looking at ことはない the same exact thing is happening. ここは危ないことはない is literally “As for here, the concept of danger does not exist”, or better yet “The concept of this place being dangerous does not exist”. Notice that こと is actually a “thing” but it’s a conceptual thing, not a physical thing. こと encompasses a state of something, which encapsulates more than just a single thing - but this isn’t really necessary to explain unless we’re wanting to find the differences between こと and の or もの.
Edit: I just realized your question only had to do with if the adjectives are special types or not, which I’m not even 100% sure if they are or not lol… but to my knowledge they aren’t. Either way… hopefully my answer was helpful in some way even if it wasn’t your question.
I don’t know about the correct classification and such, but on the last point, cannot think of other words that end with hiragana ない. Maybe if you consider 仕方ない to be an i-adjective, like JMdict does, although I think of it as just a set expression.
This is a fun question!
Regarding 危ない, it is special only in its relative rarity. This dictionary entry says 危な is the stem of 危なし. 危なし is a classical Japanese adjective in the conclusive (dictionary) form (search ク活用 on that Wiki page). Modern inflection replaces ク活用’s し with い. So 危なし became 危ない.
I have not looked into 少ない deeply, but a quick dictionary search suggests to me the answer may be different than for 危ない.
One way to search is with the query #adj-i ?*ない in Jisho. This turns up many results, but most of them are negative inflections of verbs counted as i-adjectives by the backing dictionaries Jisho uses, not always the case in Japanese-Japanese dictionaries. Words such as 勿体ない are considered adjectives, but the ない in that particular word can be written as 無い, unlike 危ない which is incorrect if written 危無い.
[EDIT: fixed Jisho query; search #adj-i, not #adj-ix.]