Mixing brands of metal bodies?

Has anyone had any luck before swapping around and mixing uppers and lowers from different companies? I have this burning itch to have full M4 trades on my S-system. And have found a G&P body kit with full Colt trades. I would use the G&P upper, but have heard that the S-systems have a slightly different upper because of the long top rail. So I would try and use the G&P lower with my stock D Boyz upper if I can. Has anybody had any luck mixing? And if so, what brands.

s-systems normally do when made by jg and ca but d boyz made a custom rail system to use a standard upper receiver and lower receiver well at least on mine they did i compared it to a friend of mine who has a jg s-system and his had a different front design to it mine was the exact same to a normal m4a1 like my neighbors have so unless your d boyz has a weird front end design you could use either the upper or lower receiver or both if you wanted

I know my G&P upper has the real steal barrel nut threads. same as ICS or Systema. but if you wanna spend the time you can get a small triangle file and file around the threads like your rethreading it and the smaller nut should fit fine. thats what i did to mine. as far as fitting the upper to lower im not sure. and I have a DBoy S-System and the rail fits fine on my classic army upper, the metal version.

Well I guess we'll see when it gets here, because I just ordered the G&P body with full Colt M4a1 trades. I have a feeling I should have no troubles making the things work the way I want to.