Gonna set this straight, Ice Trousers proc very well for mobs that are at the max Toughs at lvl 75. I use them alot on Drg, Also you "do not" need them equipped during the said effect. After i used them and ice spikes came up i switched to homam cosciales and the effect was still active and working. I first used them on Sewer Syrup as Drg/Blu 75.
Ice spikes - Must have the trousers on all times and it last 5 min, 5 min recast Proc rate - 5 damage to Lesser collibri (at 75) every time and proc rate for Paralyze from what I notice is as good as a RDM ice spikes. Ice shield and ice trousers stack? - no, trousers is stronger since its the actual spell and will be base on your enhancing magic.