The lists are still in copyright and will have to be bought, I believe.
You will want to use Middle Imperial Romans as your list source but in actuality, the Late Imperial Romans under Julian were the ones that fought the Sassanids. Time period wise, the Middle Imperials 'could' have fought Persia in this period but didn't. You are actually using WRG 6th edition lists by the way unless you use NASAMW lists instead.
Marines are only listed in the EIR lists (Sassanid Persians are from 220 to 637 AD and fall beyond the EIR list period which ends in 192 per the WRG lists)
Marines in the EIR are the same as legionaries so you would base on 40X15mm stands with 4 figures per stand. They count as Heavy Infantry. Their morale is 'C'.
Dromedarii are listed as Light Camelry in the lists. I don't recall if they were used against the Sassanids but I would bet they were used in scouting more than in battles.
I would have a serious look at Warrior Rules if I were you. They have list rules that give a better flavor to Romans and other armies. My only beef with the rules are some list decisions made by the authors that don't stand up to historical review in some cases. The Chinese armies are allowed to remove the cost of shields for their rear ranks for all units as an example and this is highly dubious. Also, allowing Almughavars to count as shielded, when they didn't have shields may make them too powerful. They were great against historical opponents, they shouldn't be great against heavy missile fire.
The rules are fine for the most part. CB, lancers and counting part of the second rank of Jls armed cavalry may have gone a little too far in rules changes from 7th edition though. fourhorsemenenterprises.com is your source for these rules.
By the way, WRG 7th A (assuming you mean first release), is loaded with errors. You need the next published rules which reads "August 1992, fully revised" on the cover.
