I’m designing board and little bit uncertain about board form. I think that (a) cut is ok, but what about (b) cut? Can i design these both with happy mind?
As long as slot (b) is at least 0.1" (100 thous) wide, I wouldn’t worry about it.
If it’s narrower, and/or you’re still concerned, add a comment on your order. I’m pretty sure all BatchPCB orders get reviewed by a human at some point, and they’ll let you know if your design has obvious problems or can’t be manufactured. Something like this might even get caught by the DRC bot.