Since you can use silicone grease to lube the piston when rebuilding calipers, it must be OK with brake fluid.
DO NOT DO THIS. DO NOT let silicon near standard brake-fluid. It's frightening what you'll read on these forums.
And worse, someone else thanked him? And, stands back.... the logic? Oh dear, oh dear.
Standard brake-fluid (DOT 3; 4; 5.1) is glycol base. Silicone, and er... glycol ( a from of alcohol) do not mix.
The clue's in the name... "gly".... 'col' and er... alcohol. It's in the Latin. Red rubber-grease is vegetable-base, usually Castor oil, it is compatible with glycol. Silicon grease is not. DOT 5 is silicon, use silicon-grease for DOT 5 silicon fluid only. DOT 5 is the purple stuff. The only grease which may come in contact with glycol brake-fluid (DOT 3; 4; 5.1) not 5, is red vegetable-base.
Please if you don't know what you're talking about fine... I don't know too much about DIY dentistry, I consult a pro.
There's three greases used in most brake set-ups. You'd use:
a) Copper-nickel grease on metals, NOT near rubbers; NOT near fluid.
b) Red vegetable grease where in contact with glycol.
b) Silicon to non-fluid side of rubbers. AWAY from glycol fluids. Metal if you like.
Reasons: The rubber in contact with fluid will be EPDM. EPDM is the king. EPDM is near invincible with all greases, your problem is the fluid won't be. Your caliper pins will have gaiters, these likely won't be EPDM. Probably cheaper, maybe Nitrile if you're lucky. Don't put copper-grease to those gaiters. Silicon works here. Silicon grease is stable with rubbers so won't attack any. Red grease is no use anywhere away from fluid, it washes away. Copper-grease goes where there's no rubber. Or... only put copper-grease on rubber when you know for fact it's EPDM - only, it's doubtful you will know.
if you DO rebuild calipers with silicon using Standard brake-fluid (DOT 3; 4; 5.1) expect woe.. A gloop will form as the glycol reacts with silicon. This won't be good. This gloop breaks into a snow and is near inert (hence there are no easy solvents) and this will clog ABS pumps filters etc. Without complete strip-down, and an air-line through all of it, it's a pig and near impossible to remove.
As for bleed-nipples, The rubber-cap is there to help. Silicon, copper or nickel grease. Needs must, bathroom-sealant if that's what you've got?... anything to stop the wet getting in there and corroding the lot together for next time is good. It's a parallel thread, not taper. Thus worry if it's not the cone that does the actual sealing. You would put actual thread-sealant on there if using a vacuum-bleeder; bleed-valves; MityVacs etc etc. Good for vacuum bleeds, good for keeping the wet out.
As I said, just watch what you put near brake-fluid, and for similar reasons don't mix brake-fluids of different types... no matter what it says on the tin. Two differing fluids mixed will nearly always lower the boil-point. Idiots on here banging on about upgrades to DOT5.1 from DOT3 or 4 only taking into account manufacturer boil points, WITHOUT the mix, or flushing nearer a gallon through are... well, ahem, that's another thread.