Some do and some don't. Over at the wiki...
http://subterfuge.wikia.com/wiki/Subterfuge_Game_Wikia... we have listed them as either static abilities (like 300%) or additive abilities (like +25%)
To be more specific, the Admirals global abilities do stack as you have suggested, though it is only on subs containing no specialists. For those subs, they run at the speed of the specialists in attendance and don't get a bonus.
Several versions ago of the game, the Admirals were crazy powerful because they buffed everyone, though at a lower amount... so you could get Smugglers running at almost 6x and Helmsmen at 3x or 4x.
It may feel a bit incongruous that the Admiral does buff subs carrying other specialists, but it's just too powerful.