Heh, an army of Harry Potters? Oh, that's going to be a real surprise, even if they're only restricted to the spells Harry knows at the end of Fourth Year, they're still going to be quite a surprise to the on-coming forces and mercs. I suspect there's going to be quite a surprise for a lot of folk when they go into action.
Of course, the real fun will come at such time as Harry returns out of the RoR with whatever forces can come with him out of the Room. Another question is the time differential between where Harry is now and outside the roon. If the room allows a lot of time to pass inside for comparatively little outside, there's going to be some very surprised folk when Harry exits the room.
Author's response
Yea, I'll allow earth to be located in space as well. Just add some more fire-power to Harry's army. Don't worry about his skill level, I placed him on Kamino for a reason. You'll find that out soon enough lol.