In the Lost Universe novels (still untranslated into English) it is said that "Darkstar" and "Vorfeed" are gods in the mythology of the ancient alien civilization. These aliens merely named their ships after them. When the author, Hajime Kanzaka, was asked about the connection between Slayers and Lost Universe, he responded that he re-used these names because he found them "cool".
The director who worked on both Slayers TRY and Lost Universe, Takashi Watanabe, decided to run with this, and make TRY into a bridge from fantasy to sci-fi. The silhouette that appears in the last episode of TRY is definitely modeled after adult Canal (in fact, the art on the box for the 6th volume of the Lost Universe DVD has adult Canal holding an "orb", in the same manner as the silhouette that reincarnated Valgaav.)
Now, whether you believe there's an actual connection between these two world, or these are just mere cameos for Watanabe's next anime project after TRY, is really up to you. It's clear to me that Kanzaka gave the anime staff freedom to have fun and decide for themselves on whether they're united or not (after all, the story in Slayers TRY was not based on any novel written by Kanzaka, most of it was written by Watanabe instead). The fact that Kanzaka confirmed the existence of multiple worlds in Slayers, without going into specifying them much, means he clearly wanted the fanbase to create their own theories about this.