From what you describe, I think you'd like a Lorica Manica. It is the roman full arm gaurd often seen popularized by gladiators, but what actually a military used armament first. It is like the Segmentata shoulders, but in most cases, extended all the way down to the wrist. For yooue particular wants you can always shorten it to the elbow or shorter, depending on what you like.
example:
Here is a website with patterns and a tutorial on how to build one.
http://www.larp.com/legioxx/manica.html
For your piece, you might want to combine the Segementa shoulders with a Manica, to get that full arm effect. either way, it is good historical and actual armor that is fully functional and relatively easy to make. Good luck and take your time, Rome was not built in a day!
As far as mobility goes for the Segmentata shoulders, they are exceptionally mobile and comfortable to wear. You just have to make sure they are comfortably strapped and fit right, and you can barely tell they're on! And for the lions, I buy brass lion door knockers from Big Lots, believe it or not! They are about 3 dollars for a pair and look real nice and add true class and beauty to a standard militia piece!
here's some off ebay:
http://antiques.search.ebay.com/lion_Kn ... RCHQ3aSRCH