Ok, so to answer your questions, no, sliscus does not count as the 10th warrior for the Cannon. The best way to use Slicuss is an all wych army, with 1 squad of Trueborn kitted out to make full effect of his ability, (2 Cannons, rest carbines or rifles)
But anyway, lets look at the list.
HQ: He is ok, I dont think you have a list built around him, but lets put that aside and say you want to use him, since that is why he is here. Also if you are using a utility character like him, I would pick 2 of the 3 abilities and use only them. Trying for all 3 is hard to do in such a low point battle.
Troops: First off, keep that blaster. Its not about duality, but its about the *just in case* senerio. Our AT is plenty, but unreliable. Having a blaster in warrior squads not only gives you a solid shot against Power Armor units, but it gives you an AT threat out of a troop unit. Which is great.
Secondly. Why just 5 wyches in a raider??? First off the best number to tarpit is 7, and you dont have any upgrades that come from being a tarpit unit (helatrix with Ag, Shardnet) No Haywires, so your not dread tarpiting, and you have almost no numbers, so your not drowning units in attacks. Actually... 5 wyches arent even killing 1 marine on the charge... useless points.
Elites: While you 2 solid choices, your severely top heavy in points values. Look at your Elites, its a third of your army, and your only rocking two troop choices, one of which is lacking. All it takes is killing the two troops choices, and you cant win.
HS: Keep all lances, The Dissie has lost its apeal alot... plus you can never have enough lances.
So what you are looking at is what is more important??? the Trueborn or the Incubi??? Personally I would say the Incubi, because your army lacks a solid punch. But if you want to use Duke to full effect, the TB are the better of the two choices, but they have to be kitted out right. So I would say 4 with 2 SC and 2 Carbines.
Now with our new amount of points, lets get some new troop choices. I like duality, so 5 warriors with a blaster in a venom is a steal. Lets throw two of them in, And how about a squad of wyches, so you can get some use out of that re-roll drugs on Duke. So how about this list.
XX- Sliscus
151- 4 TB w/ 2 Cannons, 2 Carbines. Venom w/ Extra Cannon, NS
125- 5 Warriors w/ Blaster, Venom w/ Extra Cannon
125- 5 Warriors w/ Blaster, Venom w/ Extra Cannon
218- 9 Wyches w/ HWG, Shardnet, Helatrix w/ Ag. Raider w/ FF
115- Rav w/ FF
115- Rav w/ FF
Total: 999