If I were to do an Endless run let's say to floor 1k but the cut it up to have it also be for the 250 floors speed run or 500 etc I assume this would go against the rules correct?

It wouldnt count at all. It doesnt mean "do 250 floors" or "do 500 floors". It means until floor 250 and until floor 500. So 1 - 250 / 1 - 500 / 1 - 750 etc

And speedruns always starts from the selection screen. At least the record. Timer starts once you confirm new save slot/overwrite save slot

Yep, it gotta be the first 250 floors consecutively.

Thats what I thought just wanted to clarify haha