How to make different setup chess variants?

Normal setup chess - placement on 1-3 rank, 5 types of pieces, no matter how many pieces of each type, 39 points to setup. That's easy. "Setup material" to 39 and you got this

But how does setup work in Ashtandrez? Placement only on 1st rank, only 2 types of pieces, and only 1 piece of each type. Another weird thing - description says "setup 3,4"

How to make a variant with setup rules like in Ashtandrez but with possible placement on first 3 ranks?

Also, how does setup chess work on a bigger board?

I won't pretend to fully understand all of your questions, so I'll give my best shot. You can change the rank limit in the "edit position" section of a custom Setup Chess game, and although I have no idea if there is even a "default" for larger boards, you can change it however you like.

As for combining Seirawan Chess with Setup Chess to mimic Ashtandrez: at this moment, it's not possible, but I suppose what you could do is have a custom position with the 2 dropped pieces already on the board before the game; after all, the default for a normal 8x8 board is 3rd rank as the furthest you can place, so space isn't an issue; as for the points, just set that to however much the pieces in Ashtandrez add up to.

I could see there being an issue with uniqueness though, since players could likely just spam knights or rooks like in standard Setup Chess.

Ah, okay. Didn't see this thing earlier.

On a bigger board there's a choice of setup rank limit between 1 and 14

And to make setup like in Ashtandrez you need to change "setupComplete" to "true"

Always answering my questions myself)

Probably this post can be useful for another variant creators

Can we change the drop rank limit in crazywan or even crazyhouse though?

You can always work some magic with fen editing

I'd suggest taking a look at ashtandrez and it's fen + pgn to see what some of the options used are