@sirrian has said that guilds need to elect to register. This is to keep only "active" and "interested" guilds in participating in the "Pre-planned" battles. What I would hope this means is that the devs will do their best to pit Guilds of similar power (which I assume will be based on Guardian Levels and Number of Members) against one another.
SO, all of that to say, as long as other 1 man guilds are clicking to compete, you should be fighting a fair opponent. If no other 1 man guild clicks, you might find yourself against a 2-5ish member guild.
If done well, you should be able to accomplish... something positive from participating in Guild Wars, but not at a rate comparable to a guild of 30, but you seem comfortable with that choice as you stated in your OP.
I hope that gives you some hope for Guild Wars!