Can I safely operate a PLL below it's "minimum" RF frequency?
Analog devices provide enough range from their pin-compatible PLLs: -



This little lot cover 50 MHz to 7 GHz.

FYI I've operated the ADF4111 at 70+ MHz and it still worked but that's pushing it I reckon.

Most (if not all applications of PLLs) are on a limited target bandwidth. However, AD do say that the widest bandwidth application is probably cable-TV tuners where it might be about 400 MHz. That's one side of the reason.

The other side of the reason is that VCOs (to maintain their low jitter characteristic) only have as much push and pull range as the application demands. Think of it another way; if the VCO control voltage range was 5V and it gave you a 400 MHz coverage, +/-1mV would give you +/-80kHz of jitter - probably far too much for most applications so, VCO's have a limited range.