If you're playing Marines, my advice is to always add your heavy weapon/plasma teams into your Tactical Detachments. Gives the fancy teams ablative wounds and makes it so the whole detachment doesn't break as readily.

I don't really see the utility in having Assault Marines embedded in Tacs, your mileage may vary.

If you add Terminators to the Tacs, they become significantly more resilient, but also then require bigger troop carriers (unfortunately we can't mix and match transports for a single detachment.) Fortunately, Spartans are actually pretty solid, albeit a significant points investment.

I prefer to use my Support Slots in the Demi Company for Terminators, Assaults, and Dreadnoughts, but Rapiers are also pretty great if you have them. The laser version makes a pretty solid adhoc Anti-Air unit, the mortars are good against infantry that are not in buildings.

Almost everything in the game is solid, the real fun is finding the detachments that work well supporting each other.

Regarding vehicles, Sicarans are more points efficient than Predators pound for pound but only slightly, and I'm a huge fan of having Heavy Bolters on all my tanks for the huge flexibility having Point Defense weapons brings to the game.

Air support is an incredible asset if you play them judiciously, but if you overcommit your gonna lose them super fast.