"Strongest country in the world" is propaganda spread by Saint Rat and Dorry/Broggy
Nah.
Big Mom's intelligence network is second only to the World Government's, and probably superior on some levels.
Linlin wanted Elbaf because she knew the Giants have the greatest army in the world.
Through the series, we've had Giants fight with both allies and enemies (in Hajrudin's case he was both).
Wouldn't be surprised if the Giant Army split up at some point. IMO they're strong to be pure allies for Luffy. If Luffy gets all of Elbaf behind him, the EOS fights will be boring.
I could see Teach recruiting the worst Giants their society has. Cross Guild goes for Mercenaries. People who believe in Nika go to Luffy. Marines will definitely get a good number, just because Akainu's Absolute Justice and militant ideals are very attractive to the more violent giants, who still want order and dislike piracy.