I'd like to see something like this to start.
Schwartz/Schenn/Thomas
Fabbri/O'Reilly/Tarasenko
Steen/Bozak/Perron
Maroon/Barbashev/Jaskin
If I were coach, I wouldn't start Thomas off in his first pro game on The Blues' top line. He's only played on The Junior level with teenagers, and not-very-fast or skilled 20 year olds. The pro game is a LOT faster, with less time and space to operate. A rookie also has to be learning his new team's offensive and defensive systems and individual duties. I wouldn't also saddle a rookie with the pressure of having to perform on his team's #1 line, when he's first trying to get himself oriented. I understand that he'll be doing some of that during camp and the exhibition schedule. But real standings-counting games are VERY different, when it comes to feeling pressure. It's similar to moving up from bantam all the way to Junior A, back in the old days. There was a lot more pressure, and the game quality was at a different (higher) level. They don't call it a higher level for nothing.
Let's let him get his feet wet on the 4th Line for the earliest part of the season, under no pressure to be a defensive whiz, and also no pressure to score points. Let's let him get the feel of the game, so he can then, afterwards, work on easing into to getting familiar with his new linemates in both offensive and defensive situations. If we want him to start at centre, so as not to get sidetracked feeling comfortable as a winger, and having to start off learning the Blues' system centre position later, he should start on 4th line. He can move up to Line 3, later (after 20-30 games). IF it's decided that The Blues will have a traditional defensive/puck possession 4th Line, playing only few minutes, The Blues should then start Thomas on 3rd Line, but at RW, so as not to give him too much to take in during his "orientation period.
Personally, if I were coach (I've coached kids, but not teens, but played till 17), I'd start Thomas at 4th Line centre. I wouldn't want him getting 20-30 games at RW, and losing that experience at centre, as I'd want him to be as comfortable as a Blues' system centre as possible, after Game 65, during the playoff seeding run and through the playoffs. I can't see The Blues missing the playoffs this season. And having Thomas as a significant contributor will be needed. The more comfortable he is in his position, The Blues' system, with his wingers, and with his regular shiftmates on The PP and PK, the better.
Teams that win The Stanley Cup, or go far in The Playoffs, are usually those whose forward lines, defence pairings, and special teams units, have played together most, and gelled together. Finding those optimal groupings happens from testing through time playing together.