Rio Grande do Sul had a strong populist political presence which the coupists could fear; neutralizing it and controlling the nationally-important state capital, Porto Alegre, were their goals. An equally strong conservative presence had elected the present governor, Ildo Meneghetti.
The Army had five divisions, plus, as an afterthought, the 5th Infantry Division/Military Region in the two states to the north. It was a weak force and the states were important as a buffer zone through which the IIIrd Army could invade or be invaded by the rest of the country.
Paraná and Rio Grande's governors were pro-coup, the latter an active conspirator. On his side were the Brigada Militar (just the local name for the Polícia Militar), the IIIrd Army and the 6th Infantry Division with generals Galhardo and Adalberto. Generals Poppe (3rd Infantry), Saraiva (1st Cavalry), Camarinha (2nd Cavalry) and Garrastazú (3rd Cavalry) had a roughly similar profile, anticommunist but bound by personal ties to Kruel, Goulart, the Minister of War, etc. Silvino (5th Infantry) was a committed legalist. As a respected war hero he's still cherished in the Army today despite being on the losing side.
Colonels and other middle ranks weren't necessarily conspiring but upon the outbreak played a central role in driving the rebellion in the 3rd and 5th Infantry and 2nd and 3rd Cavalry. Pro-coup sympathies were also present in the 6th Infantry, though legalist officers were numerous in Porto Alegre. There's not enough information on the 1st Cavalry.

Silvino was in Rio de Janeiro and hence the coupists seemed to be on the advantage. Galhardo and Meneghetti didn't proclaim their side but quietly moved. The governor opened credit, recalled reserve officers and requisitioned fuel and communications.
The federal government moved. It announced Adalberto's replacement with general Ladário, commander of the 1st Military Region in Rio, and that Galhardo would have to move to Rio to replace Castelo Branco at the General Staff. Ladário would then by seniority assume the IIIrd Army. Thus three troublemakers would be knocked out in one go. This arrangement had been decided some days earlier to get rid of Castelo.
In addition, Ladário's plane would stop in Curitiba to drop Silvino.