Question: I am playing against an Imperial Guard player at 1500 pts. He has ~ 7 lascannon squads + an assault cannon squad. He just sits there and shoots at me while I try to advance.

I have since switched away from anything coming from reserve since my army would be eaten piece by piece. Instead I am going for a lot of troops:

* 3x Rhino (with Storm bolter) + 10-man Tactical squad (heavy bolter, flamer), sergeant has storm bolter
* 5-man Scout squad (all with combat blades)
* 10-man Assault squad
* 5-man Devastator Squad (3 lascannon, 1 heavy bolter)
* Librarian in Terminator Armour (Avenger, Force Dome) + 5 Shooting Terminators
* Dreadnought with Multi-melta
* Dreadnought with Assault Cannon

Since most Imperial Guard troops have 5+ armour, any hit I make with the storm bolter or heavy bolter means death. Since each Rhino has a sergeant and a flamer guy, I can move 6" and still fire 4 shots (+template if I survive long enough). Is this a viable tactic to help my main force get into close combat action?

Answer: 
List tailoring is a bit of a Faux Pas in miniature gaming. You, shouldn't build a list specifically to beat a single opponent. Massing heavy bolters would be a step down on that path. Heavy bolters are also a fairly limited weapon, since they are nearly useless against most armor. A better weapon choice would be autocannons or asssault cannons. They strike a balance that makes them good for anti-infantry and light anti-armor work.

I would say that your current list does lack in long range anti-infantry fire power. This would make you easy pickings for a foot heavy guard list. Here are a few units that you could add to your army to try to address that:

* Predator with Autocannon and Heavy Bolter sponsons. This is a cheap option at 85 points each. They bring front armor 13 and 2 ST 7 shots and 6 ST 5 ones. Taking 3 is a good use of your heavy support slots.
* Dreadnought with two twin-linked Autocannons. For 125 points, you get 4 ST 7 shots with re-rolls. This is a great fire support unit as it will also give your guard opponent trouble if he decides to mount his army up in Chimeras. It's fairly common to see 2 or 3 in a Marine army.
* Speeder with Multi-Melta and Heavy Flamer. Not specifically long range, but they are fast and can deep strike. You can drop them right on your opponent's infantry squads and use that Flamer template to kill plenty of guardsmen outright.

For your tactical squads, Missile Launchers or Multi-meltas should be your choice 90% of the time. You'l get the most use out of them. You should give your sergeants either power fists or a combi-weapon. You already have plenty of ST 4 firepower in a tac-squad; you need your sergeants to cover your weaknesses.

Another completely different approach is to load up on flame template weapons, mount up in drop pods, and land in his face. He won't ever want to run foot guard again after that.
If you want to turn the tables on him a bit, take a couple of drop pods with some flamer tac squads and a dreadnought with a flamer / multi-melta. This would force him to choose between starting with his army on the table (and then having it horribly burned by the drop pod assault) or starting his army in reserve (to dodge the drop pod assault) and then giving your main force the time it needs to sprint across the table.