You seem to have a misconception regarding the recruitment of neutral creatures. While those initially generated on the map as guardians of objects or artifacts are unlikely to join, at least I haven't encountered that in practice, we're talking about neutral creatures that appear on the map as a result of weekly events. For example, playing as a Fortress hero, when the Stork week arrives (which doubles the growth rate of Swordsmen), I can recruit as many Swordsmen as I want. I'll specifically load up my hero with troops, and every other stack of Swordsmen that appears as a result of the Stork week will ask to join me, and there will be plenty of these stacks on the map. Thus, it is quite possible to recruit 1000 Swordsmen into my army without having any growth for them at the start, and it is precisely this that the rule about recruitment prohibits.