You don't to split them. Just make sure you didnt' get any media in there.
Assuming they aren't connected to a brake hose I would use air to get the pistons out. A 6 piston caliper can be a little tricky. I try to get them all out a bit and then work one at a time. They make special pliers for this (someone posted a photo here not too long ago and it was news to me) but, I've never needed much other than air or, hooking it up to a brake line.
You're putting in new seals? Brake fluid works. I have some grease that I received with some caliper rebuild kits long ago that I use. Its pink. Kinda feels like silicone. Not quite sure what it is.