Can you trump the first trick in Spades?
No trumping the first trick is definitely a house rule, insofar as rules for the game actually exist (there are no official ones). It's hard to say exactly how common it is; for what it's worth I think most people I've played spades with used it. But I haven't found it listed as an actual rule in any of the versions of the rules I've found. So sure, you could call it "just" a house rule, but at the same time, there are obviously people out there successfully playing spades with it.

So can you? Depends who you play with. And honestly, even if we could take a quick representative poll, it doesn't matter that much what most people do. It matters how you and your family want to play. If you want to try to talk them into your version, go for it. Or you can just play their way; it's not going to drastically warp the game.

The real problem you had this game was that different people had different versions of the rules in their head, and you didn't find out until you were actually playing, not necessarily that anyone's version was clearly right or wrong. You can avoid the dispute simply by settling on a rule before you play; no need to get into a big argument about what the correct rules are, or annoy anyone by flat-out stating that your rules are the rules.