Pretty much like the title says, should questions about Tokusatsu series (Kamen Rider, Ultraman, Sentai series, etc) be included/can be asked in or should it have its own subdomain such as tokusatsu.stackexchange.com?
Tokusatsu shows are by definition live-action shows. Live-action as a medium is broadly off-topic here. Specific live-action shows might still be allowed if they're based on an anime, manga, or other similarly on-topic source material. We've allowed questions like "What are the differences between manga X and its live-action adaptation?". In practice, that means that most questions about the live action versions of many series e.g. Hana Yori Dango or GTO would be allowed here. Tokusatsu shows based on manga are no different, so e.g. Ambassador Magma is basically fine since it was originally a manga.
However, if there's no anime or manga version, then it's clearly off-topic here. If there is an anime or manga version, but your question is highly specific to the live-action version, it's also probably not a good fit here. For instance, if you're asking about a plot point which is only part of the live-action, it probably doesn't belong here. We haven't had to work out the details here because it hasn't been a problem thus far, but as a general guideline I'd suggest avoiding anything which only makes sense in the context of the live-action version.
Most of the series you mentioned do have manga versions. Kamen Rider has at least 10 different manga series which cover most of the original and quite a bit more. Ultraman has several manga versions as well including a sequel to the TV series. The Super Sentai series has a few manga versions as well. So a lot of questions about these very popular tokusatsu series technically fit on this site, but they aren't universally on-topic.
There are several other SE sites which probably cover more about tokusatsu shows than we do. Most of these shows would be on-topic on Science Fiction & Fantasy. They'd also be on-topic on Movies & TV. They might be better places than here to ask those questions.