'Legendary Cards'

Lol, using the term 'Legendary' from magic.

Anyways, what happens if I summon divine fowl king/grandmaster or the six samurai, and my opponent summons another one. What will happen?

Will my one get replaced, or opponent can't even summon?

For anyone who doesn't get the question, Divine Fowl King Alector and Grandmaster of the Six Samurai has effects that state only one of this card can be face-up on the field at any time.

for 6 sam. opponent can also have 1 since ->"You can only control 1 "Grandmaster of the Six Samurai" monster."

Divine Fowl King Alector shuld be opponent cannot summon/ special summon another if u already have 1 face up.-> "Only 1 "Divine Fowl King Alector" can be face-up on the field at the same time"

opponent cannot summon.

if he summons 1st, u cannot until either players' card leaves field.
yup jadow is correct

Hmm in terms of your question, Grandmaster effect states:
[[Lore::You can only control 1 "Grandmaster of the Six Samurai" monster. While you control a face-up "Six Samurai" monster, you can Special Summon this card from your hand. When this card is destroyed by your opponent's card effect, add 1 "Six Samurai" monster from your Graveyard to your hand.]]

It states you can only control 1 Grandmaster of Six Samurai, which means your opponent can also summon Grandmaster to the field.Theres nth wrong with that.
However, in the event that you wish to have 2 Grandmasters on your field,
I checked the rulings for Grandmaster. It states with relation to hero mask:

Hero Mask: If you select "Grandmaster of the Six Samurai" and change its name with "Hero Mask's" effect, and afterwards you Summon another "Grandmaster of the Six Samurai", when "Hero Mask's" effect disappears in the End Phase the Summoned "Grandmaster of the Six Samurai" is destroyed because you cannot control more than 1 face-up "Grandmaster of the Six Samurai".

So basically if u alrdy have a Grandmaster on your field first, even if you summon another Grandmaster, the later one will be destroyed because only 1 Grandmaster can be face up on your field.

Hope it helps =)