If I were to bring a couple of Trucks, would it be a good idea to also bring Sentinels just to increase the number of vehicles my opponent has to deal with?
That is a highly non-trivial question. If you play at a higher points limit, that would be a good idea, but I'd take sentinels for what they provide, not just to take more vehicles.
Sentinels are heavy weapons teams (HWTs) on stilts. Sentinels can be cheap brigade filler (w/multilaser), provide anti-elite firepower (armoured w/plasmacannon), antitank alpha strike (2x w/missile launcher and hunter killer missiles) or deny deep strike (scout w/heavy flamer). They're still rather squishy though. Not as squishy as the achilles ridgerunner, but also not as fast. If you want them to move before the game, use the scout sentinel (gets a scout move). If you want them to be slightly less squishy, use the armoured sentinel (no scout move, but has +1 toughness and a better save)
If you can spare the heavy support slots, you may get more mileage out of brood brother heavy weapons squads (unit of 3x HWT) with mortars.
For the points cost of a sentinel with multilaser, you could get a min squad of acolytes or a heavy weapons squad with mortars.
Of course, all three units fill different roles.
As for the other GSC vehicles:
- The general opinion on Goliath Trucks is that they're overcosted and too squishy for what they do (transport a squad which could be ambushed instead, provide 2 autocannon shots and 3 heavy stubber shots), but your mileage may vary. They're still great to get squads which can not be ambushed due to the ambush limits across the table. Problem is that the same points can net you two 5 acolyte squads + equipment.
- Goliath Rockgrinders are a bit weird: They are good anti tank in meelee and can carry a heavy weapon. Problem is they won't get to use that weapon once they get into meelee. Transporting 5 acolytes and a character is icing on the cake. The cheapest Rockgrinder (w/seismic cannon) costs as much as two bare bones neophyte squads.
- Then there's the brood brother leman russ, which has no transport capacity, but a big gun and more staying power. Can take sponsons for even more dakka. The problem is: Leman Russ Tank Commanders from the Astra Militarum Codex can be taken in an allied brood brothers detachment - they're the same unit, but add tank orders and a better ballistic skill on top.
- Achilles Ridgerunners are fast but squishy, even more than sentinels. The mortar is not as points efficient as brood brother mortar HWTs and firing indirect wastes the heavy stubber shots. The missile launcher + flares is the take all corners option (read: okay vs infantry and tanks, but not the best at either), while heavy mining laser + spotter provides the only 42" ranged mining laser in the army - but at that point cost, you may as well use Atalan Jackals with demo charges instead.
Which of the units named is best for your list depends on your army composition, your playstyle and your local meta (read: what other armies you face at the friendly local game store (FLGS)). At 750 points, you usually want some cheap bodies to sit them on objectives, some hard hitting unit to remove the enemy from objectives and a bit of anti tank capabilities to counter the occasional leman russ, dreadnought or other vehicle. Also, a bit of utility can help.
Looking at the list of units you provided, we can see that
- Neophytes are your objective holding bodies (acolytes can also do this too, but are more squishy)
- Abominant, Aberrants & Locus are hard hitting meelee units (the blob of hand flamer acolytes can be scary too, but they need to be ambushed to offset the low range)
- Abominant and Aberrants with hammers can double as anti tank
- Iconward provides buff utility
You seem to be a bit low on anti tank - a unit of 5 Acolytes with 2 Rocksaws could fill that hole/role. Alternatively, Neophytes could be equipped with mining lasers, but the low range would make ambushing them a must - and they can't stand on and hold an objective when they're in reserves.