Is there a limit (and if so, what is it) to the size (mass) of terrestrial planet that could be "captured" as a moon by a migrating gas giant?

I'm writing a novel where a colony ship crashes on such a moon. My research tells me that a moon around a gas-giant is not likely to be larger than 1:10,000th of the mass of its parent. If this is true, an Earth-like moon is unlikely to form around anything with less than 30 Jupiter masses, which puts the parent planet in the brown dwarf range. Not what I want. 

So, can I get round this by having the gas giant migrate into the inner system, snaring a rocky world approximately the size of Earth, as it goes? 

I'm not too concerned about other issues - I'm happy to fudge tidal locking, and have the gas giant only have a couple of other moons to avoid tidal heating, and stick the Earth-like moon 10ish million kms out to avoid the worst of the radiation. But I feel like I can't fudge the mass of the related bodies. 

Any help, or related thoughts, would be greatly appreciated.