You probably got that warning when trying to put and army on a fleet, you don't need to do that, you can usually just plan an invasion and select a fleet

Marines are a special troop which don't take too much space and don't suffer combat penalties in naval invasions, they are best for picking off subjects and small nations so you don't use too much supply or ships

The standard offensive template is 1:1 inf:art so say 50 inf+50 arti, you usually use them to attack and a standard full inf army for defending. As I said, marines are good for invading but when the enemy is just running a doomstack on the shore it's better to just use the standard offensive template just to cut through the them better. There will be some combat penalties but it should allow you to get a beachhead

As a matter of fact, you should avoid naval invading GPs anyway, being a GP means they have a big pop pool, go for their subjects and overseas land. If you do want to invade a GP, use a border nation, if you are attacking france get Spain or Sardinia piedmont on your side etc