I have a few friends who have used "gentle parenting" techniques with their young toddlers. It's a small sample size, but without exceptions the kids are all showing some really challenging behaviours. Still pushing and shoving at the age of 4. Struggling to be kind and gentle. When the parent "deals" with one of these incidents, the story goes something like this:
"does xxx looks like he enjoys being hit" (xxx is in tears, with dc of gentle parent on top of him)
Child responds: "yes" with a grin
Then hits them again.
Gentle parent tries again: "no, he isn't enjoying it. Look at his face, he's crying"
Makes my blood boil.
My interpretation is that these kids are begging for proper boundaries to be put in place. Don't ask your toddler o make these decisions for himself, he isn't capable of that kind of reasoning just yet.
What worked for us was saying a very clear "no" to unacceptable behaviours. Usually followed by an age appropriate explanation ie "no pushing" or "that's too loud, it makes the other people's ears hurt".
At the age of 18 months a lot of this was accompanied by gestures. Point at your ears when they are shrieking. Show them a pushing gesture when you want them to stop shoving other kids. The first priority is to stop the bad behaviour. Kids at this age lesen through behaviour, showing, examples. Not so much cognitive reasoning.
Nothing you do will eradicate challenging behaviours, but being clear and natural in your responses (rather than forcing yourself to be gentle just so that you never raise your voice) will give your child a road map with which they can navigate the world and society.
To be fair to you, at 18 months you are in the transition from having a lovely innocent baby, to having a young toddler who is capable of being inappropriate and annoying to you and others; but who is at the same time incapable of reasoning and logic. It's a really difficult transition for all of you.
Be very clear with them, and make it simple. Above all, don't hand over the responsibility to parent to them. It isn't fair.
And then, once you have chosen your form of parenting (raising your voice, saying something, putting them in the thinking corner or whatever it might be) - don't drag things out. They forget things really quickly so the learning has to be instant or it will be lost.
I think at 18 months I would plonk my toddler on he floor and count to five, while telling him what he had done wrong ("we don't whack our newborn baby sister with Lego"). Then forgive and forget and move on. It's not easy! And we don't always get it right.
Trust your instincts. Read fewer parenting books (you might not be doing that at all
)