Well since 8th edition allows you to split a squads fire across multiple targets I would say being able to go after both infantry and armour/monsters is a good idea.
I guess that if I choose to split roles in squad I should choose weapons which are completing each other (similar stats, range..)
As far as ratio goes its all about the role(s) you want the squad to have and how your going to move them on the battlefield. Since you intend on having them at close or mid range most heavy weapons aren't going to be that worth it. Overall I'd say take 4-6 combi weapons, a single heavy or special weapon, and the rest as special issue ammo.
What minimum amount of one type of weaponry is required for it to be effective? With 1 heavy flamer and 4 combi-flamers it will be good anti-horde combination but with only 2 combi-flamers it might be much less effective.
Thinking out loud as I intended to combine flamers and meltas in one squad. Which would mean 1 Heavy-flamer, 2 combi flamers, 1x multi-melta and 2 combi-meltas, rest with bolters. Which might not be very effective ?
If I intend to build anti-horde squad they should have one type of weaponry so they get more hits and kill more models?
But in case of anti-elite-infantry (CSM, SM etc) it would be enough to have lets say 2 combi-grav and grav cannon because they would be fighting against less models.
As I wrote before I will have 4 full squads so there's plenty of combinations.
Squad one - flamers - anti horde
Squad two - meltas anti armour/monster
Squad three - anti heavy - plasma/grav combination
Squad four - heavy bolters, bolters (because I love heavy bolters)
That would cover a lot. And devastators would get lascannons, plasma cannons, missiles and all that long range weaponry. Or are devs better with some heavy weapons then sternguard ?