Dress code

I have never heard of capris being a no no. What you will encounter is that the Europeans/locals tend to dress up a bit more than the standard tourist especially in the larger cities.

As someone who goes to Europe every summer, quite often in Aug & Sept as this year, you will find the weather cool and hot from Paris down to Rome. Be sure to take clothes that can be layered for comfort. However, the weather in Europe is somewhat unpredictable as it is here in the US. I always carry a light weight rain jacket in my day bag just in case.
You indicate you are covering a lot of ground in your travel - I hope you have sufficient time to pay it all justice.

There are no "no-no"s, except for shorts and bare shoulders in churches. No one in Europe cares what you wear. And when you see what tourists from other countries wear (picture umbrella hats and white knee socks,) you'll realize that choosing clothes based on some perception of fashion standards was a waste of time. Wear what makes you comfortable.

OK, so so far you have answers from 3 guys. I will try to change that. I cannot speak to Paris, as I have only been there in April, when capris were not an option. In Germany and Italy in late summer (September) they are fine for most purposes. I take a pair (or two) of capris, a pair of long pants, and a skirt, and see what the weather brings. Dresses are always fine; you can add a jacket if it becomes cold It should still be warm in Italy; Germany and Paris could be anything.

I wear the same thing no matter where I go - Jeans, shape-ups, and whatever shirt feels right for the temp. and weather that day. Add a jacket of choice. Some like skirts and sun-dresses. The weather can be hot or mild or chilly in Paris or Munich. Best guess is mild.

Elvia, capris will be just fine! Will your trip be in the order you listed with Paris your first stop and Rome your last? As others have said, be sure to be able to layer because the weather is so unpredictable, more so the last couple of years. Since I look terrible in capris, I don't take them. I take one pair of pants with 3-4 tops, one skirt or dress, a cardigan, a raincoat and a pair of shoes. That of course is in addition to the pants, top and shoes I'm wearing on the plane! Depending on when I'm going, one of the tops is long or 3/4 sleeve.

Doug is right that some churches (particularly in Italy) require you to cover bare shoulders and wear something that also covers most of your leg (ie, no shorts, super short mini skirts, sun dresses or mini dresses). If you plan to visit a church, best to avoid wearing those things but if you must wear a top that bares the shoulders, at least stash a large scarf, shrug or cardigan in your day bag that you can put over your shoulders before entering the church. I spent a week driving from Melbourne to Adelaide with 2 Parisian women and pretty much all they wore was capri pants that they brought with them from Paris. I also have relatives born and raised in Italy and I've seen plenty of Facebook photos where they are wearing capri pants. They are not a no-no. Capri pants are worn by Europeans. They provide enough coverage to get you through sightseeing and a lunch or dinner at a nice bistro or cafe while looking appropriate for a big city. No one can predict the weather. At that time of year, best to prepare for both hot or very cool temps by packing things you can layer and having a day bag that is large enough to hold a sweater if you need to shed a layer later in the day.