If I retire one brute in my acc, could I replace him then with another brute for free?

as answered in the other thread , no a retired brute remains in your team and you will be able to take him out of the retirment after every 24 hours reset.

Yep, it's stupid. If you retire a brute, he won't show his cell and he won't show on the rankings or in other brutes' arenas. He also can't retire when he's entered a tourney. There's really very limited use for that function unless you've got tons of brutes in one account.