All evidence I've seen about Radical Dreamers points to being the sequel of Chrono Trigger. It was out for the Super Famicom so finding a rom shouldn't be a hassle.
Radical Dreamers is a text-based adventure. If you don't mind playing a video game "choose your own adventure", you might like it. Based on your choices, there are different outcomes and endings. The scenarios differ considerably. Some are related to Chrono Trigger, others are just crazy random stuff. Chrono Cross is basically based off of some ideas from Radical Dreamers, and uses most of the songs(it's interesting to hear the RD versions), but the majority of CC is different. It kinda seemed like, they realized that they couldn't make RD into a full game, so they took some creative liberties to expand the story, which is what CC became. Interestingly enough, RD takes place in a different part of the CT world than CC, a region many think is located between Dorino and Porre, in the forest area where Frog's dwelling was once located(in 600ad). The general art direction and the way you play reminds me of a lot of PC games from that time. I did like the atmosphere a lot. It's really dark(lol, REALLY).
All three are indeed set in Ivalice, just in distant time periods.
...So says the Word of God.
I actually don't mind that. Ivalice is like the Final Fantasy's take on the Suikoden world. You never fully explore the world in it's ENTIRETY in any of the games, just regions, and the history of Ivalice is so rich and LONG, you can basically set new titles in centuries apart, in lands unexplored, and it wouldn't conflict with the general history. It's also one of the few, if anything, the ONLY Final Fantasy world where you get to experience the world through different time periods. FFXII takes place in the "Ancient Civlization of High Technology" which is a period many FF games reference in their pasts, but in the game's present, it's considered a time of lost wonders and the world regressed since then(FFV had the Lonkans, FFVIII had the Sentra, FFX had Zanarkand and such). FFT takes place in the post-Fall of the Ancient High Tech Civilization where the world has become more of an Medieval-set place where there is lore of the ancient technology and people are excavating airships and robots here and there. FFTA and FFTA2 are set(initially) in the most modern period of Ivalice, where magic pretty much disappeared(and has become a thing of lore) and the world is in a contemporary state(basically like our OWN world today). I always liked that broad show of history.