The old man could have taken out Windu, not easily but still take him out. Palpatine was taking a calculated risk by displaying as a weaken man to get Annakin on his side once and for all.
Armor is restrictive and the Jedi wouldn't be able to do their acrobatic fighting in something that is weighing them down or stricts their movement. Robes baby!
Revan was evil who executed his will with a fine surgical scalpel unlike Malak who just used a sledge hammer to get his way.
Revan may have started off "doing the right thing" but in the end became corrupted.