Spike if you have any other questions just let me know and I can help where I can. This area its most 3/8P-043 on the 170-193; 3/8P-050 on the 201-211 and the 241, 250-291 will be 325-063, then all the full size 3/8 full size around here is 050. My common ones are:
3/8P-043 in 44, 50, 55 (44 is only on the pole saws)
3/8P-050 in 50, 55, 61
.325-063 is 67, 74, 81 in the big frame the 250/251 will be 68 mostly. Someone might have a 62 floating around for the shorter bars but hardly ever see them.
3/8 will be the same as yours already the only oddball is the 18" which will be the 66DL.
Bar mounts I'm sure you are all good on but if not 3005 mount is on everything 170 to 251, anything bigger is 3003 mount, 3002 is the big tanks.
Ummmm... lets see trimmer heads. anything straight shaft will take pretty much all the same Autocut 25-2 head. The curved shaft trimmers have primarily the Autocut 5-2. If you want non tap n'go it will be the durocuts which are just the two variations again. Curved shaft are all .080 line, .095 comes on the straight shaft. FS45/55 carbs are most common and relatively cheap. OH, on the straight shaft heads (25-2 they got these metal like sleeves that do fall out after a while and replaceable. Idk if you would just stock some parts for them or what you had in mind. BPMR7A is most of stihl, CMR6H and CMR4H on the rest for the most part. Chain tensioners are pretty universal across the common saws. Small rim setups have that different size rim. Across the board you have about 6 different style fuel caps, 4 are prolly common. You could cover your fuel filter needs with about 4 different versions. Recoil rope is 2.7mm on most small trimmers, 3.0mm is the smaller sized saws, 3.5 for the rest up to th 660's and bigger they are 4.5mm
As far as the oil the main thing is the 4mix's are much nicer to maintain if they have good synthetic oil used in them. Valves get carboned up pretty bad if not.
I can't think of anything else off the top of my head.