Purchased a tropical mimosa tree. It looked like the first picture. I waited a few days for her to adjust. I just pruned off the new shoots growing on the trunk. Too much?

Nice tree. You will have lots of fun working on that.

Are you in a (semi) tropical region where these thrive?

No. I am in zone 6a/b. Connecticut USA. I was told by the bonsai store it is a tropical. So I am only putting her outside on days above 60F and bringing her in at night

Lynda, yes you did it right.
But I don't know if it will be happy being moved in and out.

So you think I should leave her outside all summer? It still gets below 50F,10C here at night. So I was told to treat her as a tropical. Suggestions?

In Tennessee growing up we had a Mimosa in the backyard which survived many winter's with a lot of freezing weather. It was not the same as Connecticut by any means but that tree was tough. Yours may not be the exact species because the flowers are different but at least some mimosas will survive winter's. You can find more accurate information on it if you know the Latin name. Or, find out where the tree was grown.

I have a south facing bay window and the tropicals each have a goal temperature. When lows are below the goal, that tree comes in and stays in until it seems the temps will never drop below that, usually late October to early March. You would probably be a month or two longer.

It is a Leucaena leucocephala aka L.glauca. I have a sunroom that faces south that I use for my tropicals but when the temperature drops below the recommended, I have a south facing window upstairs in the house.

Should I just leave it in the sunroom until the temperatures stay low?

It really should be outside as much as possible, but doesn't want to move much. Google the Latin name and see what it says, then set the goal accordingly.

I am a little conservative on the temperature because the tree in a pot does not have the earth as a heat sink. Find a local bonsai society and see what people in your area do. Do what they say if they have a lot of old trees. Otherwise do something else.