Some preliminary findings....
Loco - Bachmann standard 4 2-6-0. DCC control. I can't comment on the OP's question about the magnets possibly affecting the decoder as on this loco the decoder is in the tender.
I chose this loco as it has always been a bit of a rice pudding special and up to now the cure has been Bullfrog Snot on the rear pair of drivers. This gives the opportunity to compare differing approaches to the problem. The loco has also been fitted with tender pick-ups which can be expected to increase drag a little.
Peco code 100 n/s track on 1 in 35 gradient
Tested with Bachmann 4 wheel wagons.
Number of wagons started successfully from a stand on 1 in 35.
Loco without mods other than tender pick-ups - 8 wagons.
Loco with 2 magnets on Powerbase - 12 wagons.
Loco with no magnets, and Bullfrog Snot on rear pair of drivers - 21 wagons.
I ran out of time for further tests, but will try fitting some other locos in due course.
Alan