MTG Salvation

I suspect the answer is the same, but the scenario is 1) activate Peacewalker Colossus ability to make Colossal Plow a creature, 2) declare Plow as attacker, 3) cast Armed and Armored, to make Peacewalker a creature and 4) attack with Peacewalker. I see the problem as being that an instant was played after the declare attacker step was over, so it is too late to declare an additional attacker.

OK, so earlier today my wife had a Colossal Plow and a Peacewalker Colossus on the battlefield. In her hand was an Armed and Armored. She activated the ability of the Colossus to make the Plow a creature, then declared the Plow as an attacker and used the mana created by the Plow's attack trigger to cast Armed and Armored, making the Peacewalker a creature, and then attacking with the Peacewalker. Is that legal? It seems somehow to break the rule that all attackers are intended to be declared at the same time (or do I have that wrong?). Thanks in advance.