1-2-3.

Well I went the full Paragon route last time, because I wanted the best ending with all the squad members alive and all possible characters showing up in the following games.

My question is, if I go full Renegade (except for the very obvious choices that would kick someone out of the ship), would I possibly lose any characters or events that would have been in the next game after if I had been a Paragon?

Don't think it's that bad. I went full renegade on my third playthrough, a few extra characters died or they died at different points or under different circumstances than on my paragon playthroughs.

Rex will be replaced by is brother

actually i don't really understand the morality system that much. i was just good guy in my first playthrough.

can someone explain it to me in detail?

What's to understand? It was fairly black and white. Be an arsehole to everyone and kill people even when there are alternatives to build up Renegade and get Renegade options in later games. Be an angel and help everyone to get Paragon options. Stay neutral to get neither... to the detriment of character relationships in ME2 (don't get dialogue options when crew members fight).

The Renegade/Paragon system wasn't a really a morality system initially although it devolved into one. It was SUPPOSED to be about different methods to achieve the same goal. Unfortunately in ME2 and ME3 Shepard just became a dick/bitch with most Renegade dialog options. That said you can go the Renegade route and still keep the whole cast alive....well going into ME3 at least.

if i go full renegade in every game, am i going to lose anything substantial aside from wrex?