How do you fill out your heavy support slots?

As much as I agree with Stella that IA lists are great etc, they are more for playing with like-minded people - i.e. people who aren't anti FW (for whatever reason(s) they have).
For pick-up games and if you play at a GW store/FLGS you are far better off using LR's from the Codex.

OP..I would magnetise them if you can, but you may have already spotted that the Demo and Exey have slightly different Mount pieces, so it's not just the Barrel you will need to magnetise.

Demos and Executioners are both very good choices but my fully kitted-out Executioner has killed far more of the enemy than my Demo..yes it's expensive but is well worth the points. Still, totally up to you if you give them Sponsons or not, but I think a sponsonless Executioner will disappoint you over time. Tanks with or without Sponsons play differently but they both have their uses.

Squadroning I would keep to standard Russes myself.

The best 3 Russes are definitely the LRBT, Demolisher and Executioner (not in order), and the Manticore the best Arty piece (well Hydras are, but they're not strictly Artillery).

Up to you what you do, and your 2 Exey 1 Demo 1 Manticore is good, and it might be perfect for your list so go for it.

Me, I find fitting 2 Executioners only happens in 2500+ lists and I don't squadron at all until I reach that limit, and even then it's just LRBT's I squadron. Hell, I take zero Russes half the time..just Artillery - just as effective overall.

I take one Executioner with Sponsons, so 5 3" Blasts for 230 pts versus 6 3" Blasts for 380 pts ..150 pts for 1 extra 3" Blast, no thanks, that a whole basic LRBT right there.
There are arguments for and against (mobility versus static, immobilised = destroyed, only 1 KP if squadroned, etc etc) but I don't often say "I wish I had taken 2".

Still, magnetise if you can, and play some games..see how it pans out and change accordingly.