>>10020557>I've never heard they had a small timeframe.It's always been pretty obvious when you think about it, they need to churn a whole toyline each year without any downtime between shows, and there's plenty of secondary mecha that just scream "I'm designed after the fact to combine with the main" and use workarounds to do so. They usually design the first mecha with extra ports for future combinations, but most of the time the next mecha adapts to these ports after the fact instead of the ports having been placed with an exact idea of what the next robot will look like.
But if you really don't see it and want actual confirmation from the designers, one of the Don Onitaijin inteviews confirms it, pic related has been translated from this site:
https://toy.bandai.co.jp/series/sentai/topics/detail/2671/ 