The Redoubt and Foundry sit well together, even though the Redoubt can occasionally be very slightly taller, but then people are different heights, and it doesn't jar too much. Perry figures are slighter than their Foundry ACW figures both in height and body mass, so I am not using too many of those in mixed units, and then only the metal figures as I don't think the plastics are that good. Dixons; I usually keep in their own units, but once on the table they all work well together.
First Corps, again are slightly bigger figures but still go fairly well in small numbers with Redoubt and Foundry. I would suggest you also check out the Old Glory figures, they are very much on a par with Redoubt and Foundry, and their marching figures are excellent. My units have had a complete refurbish since these pictures were taken, which reminds me I must take some pictures. I have gone more for marching units, but kept the variety within the units. In short I have tried to make both sides have different characteristics.
The Union troops are more formal in appearance, and the Confederates have a more mixed but still disciplined appearance. I will put some pics up to convey that better than my inadequate words.