Mixed squads of primaris, or fortis kill teams, function fine, and there are quite a few good combos at large and small squad sizes you can utilize. You can also just field primaris in their own individual units depending on how many points you are willing to spend. Also all mixed squads must have 5 intercessors, though you can combat squad to end up with weird things like 5 intercessors and 3 aggressors 2 inceptors, or something odd like that. One nice thing to remember is everything counts as troops when using combat squad even if its normally a different battlefield role.
For bigger kill teams the two most common combos are 5 intercessors 4/5 hellblasters 1/0 inceptors, this protects your more expensive hellblasters, and if you spring for the plasma inceptor, can fall back and shoot, though thats costly. The other one you will see is 5 intercessors w/ auto boltguns, 4 aggressors with boltstorm, and 1 inceptor w/o plasma, this makes the squad majority t5, able to advance and shoot with no penalty 25 wounds, and 16 power fist attacks first round of combat if it gets in melee.
If you are trying to spend fewer points reasonable combos would be 5 stalker rifles 1 aggressor, lets the squad move and shoot, good on tables with lots of LoS blocking stuff. 5 autobolts 1 aggressor works too for a bit more mobility. Generally would avoid adding reivers though as for some reason they are 2 points more expensive than on their own and really add nothing to the kill team. Inceptors I would mostly keep out of smaller kill teams too.
As far as intercessors on their own, would probably go with stalker rifles, unless your local meta is low on enemy marines and has a lot of horde players, in which case swap the stalkers for autobolts.
You aren't asking about normal vets so assuming you don't have them, but they tend to want to form slightly mixed teams if you add them later.
Edited by GrinNfool, 25 February 2020 - 09:09 PM.