There really isnt any Rule of Thumb as such because variances between Hard Beans from different Estates, Regions, etc will probably require a different roasting approach between them anyway. Ditto for Soft Beans...
Basically, Hard Beans require more thermal energy input to ensure that beans roast all the way through and the converse for Soft Beans. In terms of actual roasting time though, this might be identical given that the energy input will be different.
Of the two beans you have mentioned, the Gambella is probably going to be a bit harder than the Peru variety but thats not certain. Most new Behmor owners seem to settle on the P2 profile for most beans and then vary the Batch Size and Roast Time to arrive at a finished product that does justice to the beans and the brew quality in the cup.
Start off with small batches of 120-130g at P2, 1/4lb weight and then vary the duration from A through D and make a decision on which turns out the best. Theres no real by the book method as it happens, you just have to experiment, keep good records for future reference and enjoy the whole process. Even outcome failures are OK. It all goes into the memory bank of dos and donts for future roasts..... 8-) :)