Read through the Gun doc thread. Folks snap in half full metal factory AEGs. They shake apart factory gas guns. When they try to install miss-match parts that are sold as compatible they can't make them function. Adding on something you made Lego fashion with epoxy might end will if it's a cosmetic add on like a launcher or a stock, but not if it's the core of the item.
Anyone with the skill to home build an airsoft gun would start from scratch, if you have those fabbing skills why waste any of your cash on externals. I've seem Walther WA2000s built out of GBB pistols and even wilder stuff so this is not impossible.
Here's the hitch, those were done by folks who were in the community and experienced, folks who'd have known any of the items noted by the OP are common as dogshit on the grass come spring melt already.
If you're going to build it'll be something that is not already in every 3rd player's hands or you could get with one phone call.
Airsoft, where nothing is hurt but feelings.