Due to the similarity of these languages I’m really keen to just pot off French till I have around 20k words in Spanish ad some 8 months of only Spanish. But I work as cashier at a grocery store and come into contact with so many Canadians and French people (some who only speak French and zero English), and I just can’t help but want to learn French sooner like when I hit 10k words in Spanish perhaps? Even on of my bosses is a French-speaking swiss. Maybe I’m just being impatient as I will have already set aside next year after summer to begin French, but I could say that if I begin sooner, I could get more speaking practice from the customers and my boss. Though at the possible expense of ruining both languages if done too early. Do you guys think 10k is too low or should I just wait? Anyone who has learned both Spanish and French specifically and can share their experience, would be highly appreciated. 
I took a break from learning Spanish at around 10k known words here on Lingq, and ultimately, i don’t think it’s enough for you to leave it alone for a while. Maybe at 15,000. But if you want to, I think it would be reasonable to use 80% of your time on French and 20% on Spanish + passive listening of stuff you already know (and maybe do a bit more during the weekends to keep improving and not go stagnant)
I do find that sometimes when I take a break and resume my learning, I get better. But this wasn’t always the case. There’s some bonus that momentum adds to the table in your language learning journey. Perhaps the Spanish in your brain gets more and more saturated with input and everything just flows more quickly, the connections becoming more solid.
In short, I really wouldn’t recommend taking a break from Spanish but if you really want to learn French maybe you can dedicate 70-80% of your time to it. Just make sure you keep doing some Spanish everyday too, and a bit more during the weekends. It doesn’t have to be focused study, it could just be watching a Spanish show on Netflix (without the subtitles*) or talking to natives.