Unfortunately, I am guessing that most of the illegals will NOT be "on our side" in the gun rights debate. By and large, the vast majority of the illegals in this country come from Mexico, and other Latin American countries. Firearms ownership is usually limited in these countries; only the government and the criminals have them. It has been this way for so long that the idea of "private gun ownership = criminal cartel membership" has become, to a degree, culturally inculcated. I've had people proudly tell me that no one in their family has ever touched, much less owned, a gun; they were bragging on how good, Christian, and law-abiding their family was.
In fact, were I feeling ornery, I'd compare their attitude towards guns as similar to a Southern Baptist's to drinking, smoking, and dancing.
So, those are the people here illegally whose only crime is coming here illegally. These are the "people we want," the "honest folks hiding in the shadows." Most of them can come around on guns, realize that gun does NOT equate to bad, and join our side. That does not mean they WILL join our side. Knee-jerk reflex from their culture will be to ban guns, avoid guns, and shun gun-owners, and they'll have to break free of that.
Then there are the REST of the illegals. You read about these guys on the news. They break into people's houses for drug money. They drive drunk. They kill each other and innocents in gang wars. None of them would even qualify to own a gun, regardless of any anmesty. None of them will care if they're breaking any laws when they obtain one, either.
This is another red herring argument to get people to favor amnesty. Don't buy into it.