I do think there's more gentle ways to say you're not a fan of co-op posting, though I do agree that it's not a good idea in most cases.
... That being said if you want to start from summoning, unless you want to do something a bit more... special like what happened to Shirou, I guess there's not that much of an issue.
... In fact, summoning is a big hill in Fate RPs, so if you really want to start there I'd rather you get it done in one shot. ^^;
That being said, if there's any chance of other people being involved in the scene no go on co-op posts.