ButtonBuster

I don't have a general answer.

As to feedism:

If I wasn't serious about them, never. I'd hate to have someone get fat for me if I didn't want to stick around!

If I was serious I'd probably wait until the issue of weight comes up. It always does, in my experience. Then I'd tell her I like curves on a woman. If she reacted positively I'd say something like "You could even gain weight and it would be fine with me..." 

Then I'd let the conversation go where it goes. 

Just don't make a big deal about it, even if it is a big deal to you. 

It totally depends the intention and goal of the relationship. Casual, one night fling or quick hook up, I don’t think it matters that much at all tbh. Like just have fun and let it be. 
 

However, if it starts developing deeper or you want something more, I would say be open with it rather early. I would start by talking about what you both enjoy sexually, how you like to be touched, how your partner likes to be touched, is there any hard limits, etc. Then would be the time to bring up any kinks or fetishes. Try to make your partner comfortable, talking about ANY type of sex or intimacy can be really hard tbh. 
 

And this might be a controversial opinion, but I don’t think not having the same kinks is a dealbreaker, yes it’s nice, but if you really connect and vibe with the person it should take priority. Personally, I think this is where consensual polyamory Is great! I can’t always fulfill my partners needs, and they might not always meet mine. I’ve been in open relationships most of my dating life, and with good communication and boundaries they are great! I didn’t share one of my partners kinks and they had another partner that did, it was so wonderful and stress free! 
 

best of luck with whatever you end up doing! Stay calm and focus on communicating in the moment!