You mention that in the picture where Lord Caitanya is discussing with the Kazi about cow killing, Lord Nityananda is there with a drum. No, there is no mention of Him being with a mrdanga. He should not have a shaved head as a brahmacari but in the same feature as we always see the two, Gaura Nitai. Nityananda was brahmacari and became grhastha but you should always paint them in the Gaura Nitai style and feature you are already accustomed to doing.
Letter to Jadurani dasi, 15 May, 1974