I have Black Scorpion, Ron&Bones, Resina Planet, Black Cat and Warmachine pirates.
Ron & Bones are taller than Black Scorpion miniatures. Some R&B pirates could fit but they tend to be very tall, so you'd better forget the idea of mixing them.
In fact you can mix Ron & Bones with Resina Planet pirates and even some Warmachines pirates.
You can mix Black Scorpion pirates with Black Cat Pirates and some small Warmachine pirates.
Now you can always cheat with bases....flat ones for tall miniatures and tall bases for smaller miniatures.
For skeleton pirates I use Reaper pirates : perfect with Black Scorpion miniatures but Reaper pirates are of different sizes.
Forget Foundry, Dixon and other brands which are true 28mm (even 25mm).