Although the "fusion" of Gerry Andersons "worlds" isn't uncommon - most people don't really understand why its so easy.
It all began really with Thunderbirds - Gerry was gearing up to make more Thunderbirds, he had even started making Season Two (which got cut down to just Six Episodes) when Lew Grade, disappointed with the response to the two Thunderbirds Movies - Thunderbirds Are Go (1966) and Thunderbird Six (1968) he asked Anderson to produce "something new" and so ideas and concepts (and even in certain cases Models - which is common with Anderson productions) then got used in Captain Scarlet.
Captain Scarlet only "did okay" originally, it was no-where near the huge success that Thunderbirds was - despite its huge achievements. So, towards the end of its run - Lew Grade once again asked for something new - and U.F.O. was born.
U.F.O. being live action combined with the Andersons style and Derek Meddings AMAZING Effects work was an instant success - the Episodes set on the Moon had particularly high ratings (especially in the US) so the second series was to be set almost entirely on the Moon.
Production was started, sets and props were built - then Lew Grade got cold feet, the ratings for the final few Episodes dropped - and ONCE AGAIN Grade asked for something new.
Space 1999 was born, and a lot of the sets and models got re-used almost exactly as they were intended to be in the second season of U.F.O.
So, as you can see - the Andersons Worlds were already kinda mixed in together . . . . . .