I joined YT just so I could join this thread (so far, anyway).
I got the same email from Scale Lab, and I have less subscribers for my crafting channel than most of you seem to have - about 5 (and my main channel only has about 32).
I'm not signing up with them just yet. I did a WhoIs search, and found that their domain name seems to actually be registered to CEO David Brenner, so that's looking very positive. But, I just sent an email to Principal Media to find out for certain, and now waiting for a reply.
It just seems strange that so many people with so few subs would be getting contacted by this group (though, it IS entirely possible), that I don't want to take any chances.
Oh - and, someone posted in the thread that if someone has a low sub count, that the person shouldn't monetize. That's like telling someone with a brick-and-mortar business that since they just opened, they should give everything away for free. Monetize as soon as you get the chance, and start making something. Advertising will always drive some people away, but your content will keep people coming back if they like it. And your content will get better as your channel evolves-at least to the people it matters to, and there are always more of those people who need to find your channel.
So just do it. Let your viewership build over time. It may take a while (months or even years, depending on your content), but eventually you will not only have a loyal fan base, you'll also have a decent, if not better, income.
Just to let you know, my crafting channel only has about 5 subs ...but it only took two months, compared to the nearly a year it took to get 5 on my main, mostly-vlog channel. And it should take much less time to build it up to the 32 subs that took almost 3 years to get on my main channel. I'm no where near as dedicated to making videos as others are, but I have loyal people subbed to my main channel that are telling other people about my other channels ...and since my name has started to get out there, others are finding their way to my channels on their own. And, I monetized as soon as possible. I haven't even received a payment yet because it isn't large enough, but I am half way to my very first pay out now. I wouldn't even have a few cents built up to this point if I listened to people telling me not to monetize, let alone the half of what I need before I can be sent a check. Just some food for thought.