You could fly in on KLM, but then you wouldn't be doing a b2b and there may be some more risk doing it this way. b2b is a safer option in that if the inbound is delayed, the outbound will be too. just to note an obvious point, a b2b isn't possible with checked bags. AMS is one big terminal though, so if you did fly in on KLM you would still be in the non-Schengen departures area and just be able to walk to the gate for the BA flight - no security or passport check on the way between.

Generally connections must be 24 hours or less in order to avoid APD or incurring fare specific stopover penalties