Period Impressions is one pattern brand I suggest avoiding, unless you have a lot of experience with 19th century sewing, to the point that you really don't need instructions. The instructions are extremely vague and the markings (such as those where you match the sleeve with the armscye) are often missing. I of course had to learn this the hard way.
For any pattern, buy by body measurements, not by size. The size number has no relationship whatsoever to modern retail clothing. For women, dress measurements need to be taken while wearing the required underclothing (chemise, corset, petticoats and cage or hoop skirt).
That being said, it really helps to learn something about 19th century sewing techniques before you start. For women, the "bible" (IMHO) is Elizabeth Stewart Clark's Dressmaker's Guide, plus her Sewing Academy site which has a number of free patterns (mostly women's underclothing). If you are sewing for women or children, or just need the general techniques, her book is worth getting. Remember that sewing machines weren't in general use until the late 1850s/early 1860s, and for many years afterwards all they did was a plain straight stitch. Buttonholes need to be done by hand, as do topstitching details. As I found out to my surprise, gathering and pleating are much neater if done by hand. The Sewing Academy Forum has a section for men's clothing, but just civilian clothes, not uniforms. I don't know what source to suggest for uniforms, but I'm sure there is plenty of info out there on the internet.
Finally, whatever your pattern, first sew a mockup in muslin or other cheap fabric, fit it to the wearer and alter the pattern accordingly before cutting the more expensive fabric! Again, I had to learn this the hard way!
Bringing the