For Tredecim Scythe, base is 14.4TP with STP+21 (/sam + brutal + rajas) you have 17.4TP (14.4TP x 1.21 floored to one decimal = 17.4). As long as you don't miss the first hit of Guillotine, you're good for 6hit since 17.4TPx6 = 104.4
For Perdu Sickle on the other hand, if you want to maintain a 6hit using STP+21, you need to land at least 3/4 hits from Guillotine. Perdu Sickle is base 13.6TP, with STP+21 it's 16.4TP (13.6TP x 1.21 floored to one decimal = 16.4). Assuming in all cases you do not miss first hit; 2/4 Guillotine is 17.6TP, 3/4 is 18.8TP and 4/4 is 20TP. If you get 2/4 Guillotine and 5 hits, you would have 16.4TP x5 + 17.6TP = 99.6 TP, so close yet so far. So you would need to 3/4 Guillotine to have 6 hit with STP+21.
Asuran fists mod is 50% BRD 50% ACC
Jokes aside, you want to make sure you have capped ACC% for Asuran Fists, since it cannot double attack or crit and has a very low WS mod of 10%STR and 10%VIT. Even with 95% ACC, the chance of hits 8/8 is about 66% (95%^8 ), dropping your ACC to 90% would drop the chance of 8/8 to about 43% (90%^8 ). Since the WS mod is so low, you'd want to prioritize ATK over STR in most cases. Basically, stick to ACC rings, Dlegs, Dhead, Dbody, and no brutal earring.