Increasing the Rebel troop numbers can be done by making sure their money is $600,000 or higher. They spend $300,000 in a recruitment drive, getting 100 men.
Donating certain generic captives into Fort Sera can increase its power by +1 however (same +power value as donating 20 swords) even past 100 power. So every time I catch a HULK, ULTRA HULK, Black Gunner, Cyber Lord, Neo Mexican Cartel Soldado, Order of the Red Tide Knifeman and Samurai, Highwayman Captain, Highwayman Sharpshooter, Warlord, Raider Chief from the Celior Looter or the past 7pm bandit group in Stokke Hills, etc., that's just a free +1 power to Fort Sera.