+bunch of destroyers for coastal defense... 0 casualties on my side.

Just research max-level ABMs and defend every single city with 3-4 ABM units. You'll be invincible and the AI will not be able to do anything about it.

"IBCM vehicle: essential for winning"
Why is this?

Is putting destroyers Infront costal cities a good way to stop rocket's?

Research awac and you get a awac plane in all your airports and it have a really great radar and be used to spy on other nations. Also with awac technology you get the submarine helicopter.. It can fly very far and is really good at finding and killing enemy missile subs.

When you play as example USA it can be a really good idea to place your destroyers near greenland and on the west coast near mexico city.

PRO TIP..: Its of great advantage to place your missile defence either sam or ships at the edge of your country or out at sea on a missiles predicted lfight path. Reason is if you shoot a missile before it split in example 3 warheads then they are much more easy to shoot down.

People just having bought & starting the game with no idea of whats what will hardly be helped by the facts above!! Maybe a Youtube type tutorial would be useful here

Hmm, had a good first game as West Asia, but then tried Pacific and couldn't even keep radars up. Everyone just waltzed in with navies and said LOL fuck your eyeballs. Anyone test how much "conventional" war is "allowed" by the AI before everyone goes nuclear?

Missile Silo with 10 Megaton warhead has much further range than the normal 1 megaton one, and anything else. I use these to take out silos across the world during my end-game human extermination, although overkill.

MIRV is a must have to bypass late game SAM's with Advanced ABM.
SLBM from SSBN's are one of the cheapest options for this, and i actually find SSBN's far more useful than any other nuke vehicle, since it can launch two missiles at once and hide.
I park them in my territory for instant-restocking. Useful in lakes along with a bunch of destroyers for cover.

Best strategy is to use overwhelming cruisers to take out ground installations, to keep the enemy building to keep up with losses, instead of advancing. They're OP.
Research Cruise Missile first, or even use it as a starting tech.

But only played the AI, so can't say much more than that is the clear best thing to do.

Destroyers are very good for anti-air and anti-balistic defence. Also very good at attacking near-the-coast defences and airfields. Cheap to make.

Bombers are essential for transporting the 50 mt bomb, which is the most production effective city killer. Also for transporting ALBMs long distances they are needed. ALBMs are the most production efficient killers of everything except citys. Sams and radarstations are also nearly everytime worth their production cost to protect high value infrastructure. In ICBM alot of battles end up as attrition wars as citys can take a huge punishment before your GDP drops to hard, so that destroying the max amount of enemy assets for the least amount of own production is essential.