I'm fairly certain that they only spawn as a chance on all MMM Behemoth spawns, which I think only spawn if you're using Increased Spawns.
I'd suggest trying at the very least slightly increased spawns, which would allow for you to use Feral Ghoul Rampage(which can be fun when you have hundreds of bullets to fire).
I get random Behemoths all over the place. Seen two at once before...Hrm perhaps my machine can handle the slightly increased spawns.
I don't think I want feral ghoul rampage though. I hate fighting those things. The Reavers, especially, as they still tend to wig out and go into invulnerable spazz-out mode any time I shoot them.
Say, what weapon are you using with 20th Century Weapons? I find that most weapons using the 12.7x108mm cause some lag on my game, even more so if they fire really quickly. Maybe this only happens when you use eXcalibr, since they seem to let off a blast of fire when they hit, like the Zhu Rong.
Main weapon: AK47
Backup weapon: S&W 500
Sneaky weapon: Suppressed M16
Medium-range weapon: M1 Garand
Long range/heavily armored weapon: M82
Contingency plan: M2 Browning
I've also made my own plugin for 20th. Those are the names you see in my pip boy, RoF and magazine capacity is tweaked on some weapons(The revolver, in particular, had a magazine tweak down to 5), HP increased on weapons that I feel should be more durable, even decreased it on the M16 a bit, and the M2 is quite realistic indeed. It now weighs ~110 pounds or so, is incredibly durable and fires at roughly 450 rounds a minute. I don't get any lag from firing that beast, and I absolutely love it. Shreds Behemoths, Reavers, Albino scorpions and Vertibirds in about two seconds flat. Shreds most humanoid enemies instantly.