If FCP makes you render, it simply means one of two things …
Either you’ve added an effect that requires rendering or …
Something about the media is different from the sequence its in. To avoid rendering, everything about your source material must match the FCP sequence it’s in (codec, frame rate, audio settings, etc., etc.).
About the codecs, note that all “uncompressed” video files (aka “none” codec) aren’t created equal. If the source files were made with one codec (i.e., BlackMagic) and your FCP sequence is in another (i.e., AJA), you’ll have to render. Similarly, you’ll have the same issue if both your sequence and your source files use the same “uncompressed” codec, but one is 8-bit and the other 10-bit (or DV25 & DV50 as another example).
To determine where the difference is, when you right click the FCP bin window there is a list of things you can have it show about each file and sequence (video codec, frame rate, aspect ratio, audio codec, etc.). Check all the pertinent ones then go down the list and compare the source file specs to the FCP sequence specs.
I hope this helps!