No, stealth armor and cloaking device do not stack so far as we know. Some have claimed that cloaking in your ship will stack with cloaking from a storm or nebula but I haven't seen any proof of this.
Defense and attack modifiers do stack, though. So the defense bonus of Stealth Armor stacks with ECM.
No, ships cannot help each other cloak. Each ship is on its own as far as cloaking, and defense modifiers. No 'wide area jammers' in the SE universe -- yet.
One cloaking device will cloak anything, yes. To make things fair I suppose that higher level cloaking ought to require larger devices but that's a 'balance' issue that MM hasn't gotten around to.
[ June 19, 2002, 20:17: Message edited by: Baron Munchausen ]