I am planning on upgrading my main service panel (in basement) to a 200 amp service, I want to put a 100 amp sub-panel to the attic for central air and all the lighting on the second floor(about 18 hi-hats and 4 fans). From the 100 amp sub in attic I also want to run a 50 amp breaker feeding a small 60 amp panel in the garage. I am also installing a built in pool and want to add another 100 amp breaker to the 200 amp main to go outside for the pool panel. Someone had told me that a main panel can have up to 300% the amp rating of the main breaker because the assumption is everything will not always be running at the same time. The house is two stories and finished and running everything to the main is not practical.

Question#1 Is it o.k. to have two 100 amp breakers in my 200 amp panel?
Question#2 Wire sizes? 2/0 SEU and 2/0 copper from weatherhead to main 200 amp, #4 bare copper to ground rod and water main?
Question#3 What do I have to run to 100 sub panel in attic if it will exit basement run up 2" sched. 40 PVC to attic to 100 amp sub?
Question#4 what wire do I run to pool panel about 50 ft from main?
P.S. how is Murray panels?
Thanks very much

Someone had told me that a main panel can have up to 300% the amp rating of the main breaker

There is no such thing allowed. A panel or breaker cannot be loaded more than 80% of it's rating. A main service is sized accroding to the calculated connected load.

Question#1 Is it o.k. to have two 100 amp breakers in my 200 amp panel?
Sure, why not?

Question#2 Wire sizes? 2/0 SEU and 2/0 copper from weatherhead to main 200 amp, #4 bare copper to ground rod and water main?
Absolutley correct.

Question#3 What do I have to run to 100 sub panel in attic if it will exit basement run up 2" sched. 40 PVC to attic to 100 amp sub?

1-1/4" PVC can handle 3 #2 and 1 #8

Question#4 what wire do I run to pool panel about 50 ft from main?

If you want a full 100 amp capacity at the pool panel, then #4 copper or #2 aluminum is allowed.

P.S. how is Murray panels?
They are fine.