The lascannon heavy weapon squad exists only on paper. Well, pixels. It's a missile launcher, lascannon and (now-illegal) multi-melta in the lead.
I could split the already damaged Autocannon team, replace the missing ACs with Lascannon and attach to the Cadian squads. Then give the remaining AC team a lascannon and a missle launcher friend (I'm assuming you are allowed to blend HW squads in 8th?). Anti-tank for everyone!
And yes, mortars are great fun.
I'm not too worried about different themed units in the same squad - the entire army is a mismatch due to necessity (the shop in my hometown didn't have coherent stock or a wide range) - so the invented story is that these are the survivors of a wide variety of lost or badly beaten companies which have been amalgamated as best as possible to turn lots of little surviving units into a vaguely useful combat force. Can't be having soldiers wandering around looking for their designated units when they're perfectly capable of dying alongside others, can we!