After reading a comment earlier indicating that the level of available allies is tied to the level of your strongest girl, I dumped a bunch of manu into one of my rainbows, having left her at 61 - post evolution previously due to prohibitive expense.


Here's where the new discovery (well, new to me) kicks in.
Not only did it indeed increase the quality of random allies I could recruit, but the top levels of allies I had already recruited also magically rose to match. So now *all* my allied parties are much beefier. Here'd I'd been churning out the weakest parties in favor of new randos and making marginal gains for months, when the ones I had would've been ok.