Charlotte is prety much the same age as Nicky, and I find that I use the word no very infrequently.
I have used the word no from teh beginning though too so she knows what it is and what it means. lol she is just cheeky, like most kids, adn she will test the boundaries and experience her world...Like Jan said, about cause and effect. It is important for them to learn when they drop something from the high chair where and how is goes. It is important that she learn when she doesnt keep her footing or tries to let go of the couch that she falls down. That the glass door is infact there even though she can see through it, and she will bump her head.
I am a big believer in not moving my house just because she is now crawling around. That cat food will always be there, so I may as wel teach her not to touch it now.
when she goes for the cat food (as she does daily! lol) I say "Charlotte. No." Firmly. She understands the word, adn my tone and she stops, looks and me, pulls her hand back and then a few seconds later iwll try again. So I repeat myself, and say something like "Leave the pussy cats food alone" or "thats not for Chalrotte" ...She will inevitably try again, so I repeat again and remove her from the situation adn give her something else to play with.
After a few weeks, we are getting to the point now where I very rarely need to remove her from the food, she iwll leave it alone herself and move away. I do the same with the wine rack adn cat litter tray...power points or cables that I cannot hide, etc.
It is only through persistance that she understands "no" and understands more importantly I think, the tone of voice.
I wear glasses and find this the most challenging as she likes to pull them off, both the play with them adn to be able to play with my face (which is adorable really! lol) But I need her to learn so I remind her that "mummy needs those to see your beautiful face". Of course, when they respond to the instruction, lots of smiles and giggles and praise is the most reinforcing.

So I think that it is good that you are setting out boundaries and rules and expectations now, but keep in mind that she is only 8 months old, she will spill thigs, want to puther hands in her food and wipe them all over the table, drop food and things on the floor and whilst you may not want her to do them forever, she most likely wont, she is just learning


PS: Is it a possibility that she puts her feet up because she is slipping or uncomfortable in the chair? Or the harnes sis cutting into her legs or between her legs or something?

Anyway, hope that helps