The simplest thing for it to do, if it wants to make use of the existing information on a player’s level, is to start them at the same percentile. E.g. if they are 35% of the way down the TG leaderboard, then they play their first 1v1, start them at the ELO that is 35% of the way down the 1v1 leaderboard.
You’d still have the problem that TG ratings can be distorted if people often play with teammates who are a significantly different standard, but it would at least account for the large overall differences between the ratings scales.
Starting everyone at 1000 is an option, but it means that people lose ELO when they play against a better player who is working their way up. Transferring rating across is an attempt to solve this problem.