Remington no longer offers anything in 6,5x55 but they do still offer the Model Seven CDL in .260 Remington and it has the 1 in 9" twist which I think Remington kinda considers to be a standard in the 6,5mm bore. I'm pretty sure that would be what yours has as well. I suspect that bullets up to 140 grains should stabilize fine at that twist rate and round nose 160s might if you push them to the pressure levels of other modern cartridges but might not at the more sedate pressures typical of this round.
Extra long rounds like the Nosler BTs, Hornady SST and such would be the least likely to stabilize at the 140 grain weight or the Barnes copper bullets which are even longer for a given weight.