i play blood angels mainly. and i suggest to assume that you won't combat squad. a lascannon combat squad costs 90 points. you can get a predator annihilator for not much more than that, and you can get a predator destructor about the same price as a missile launcher combat squad.
combat squad is a tactical decision, not an efficient way to build an army. instead plan on having your tactical squad fill a rhino. if there are 5 objectives and you can claim 2 or 3 with your heavy combat squad, then go for it. all most of your army has to do is keep the enemy from surviving to take them away from you. if there is one objective in your deployment zone, leave a single heavy combat squad back.
but in terms of killing power it is much better to have a rhino or drop pod getting your marines into rapid fire distance. the special weapon designates the squad specialty, melta for tanks, flamer for troops. the heavy weapon is more of a just in case. as in i have a missile launcher just in case i ever get to fire it, or just in case every other vehicle hunting unit dies.
always use all 10 men, always pick a special weapon. a bolter can be better than a heavy weapon, but i would rather have tactical flexibility. i like the multimelta best for heavy weapons. the tactical squad is at its most powerful within 18" so that it can move and rapid fire, and the multimelta gives you another option in that range. heavy bolters are pretty great, but i prefer missile launchers. a frag missile is almost as good against troops as a heavy bolter, and it has the added bonus of being able to take down some vehicles. a plasma cannon isn't too bad either. it gives your bolter squad the chance of taking down termies.
i like best using 2 tac squads with powerfists and rhinos
the other flamer/missile,plasma,or heavy bolter
they are very tough to take an objective from in 1500 and under, only cost 500 points, leaving you with 1000 points or so to contest