7.2 A-H means you can draw ~70A constantly for up to 6 minutes. Depending on the gearbox you're using with those RS-550s and how fast you run them, they'll probably pull from 5-20A each. If this if a competitive robot, your drive motors will likely pull more current and should be what you're worrying about. If it's for demonstration or something, then I'd mostly worry about the RS-550s. In which case you could probably run that shooter continuously for at least 10 minutes or so before you had problems.
So yes, I think a 7.2A-h battery would probably suffice. Is there a particuar reason you're going with lead-acid and not, say, a
12V, 10A-h NiMH battery pack? That NiMH pack is about 60% the weight, with about 40% more A-h, after all. Of course, it costs about twice as much as well...